diff --git a/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSCatalog.java b/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSCatalog.java
index 43fc25d7a..6b0693aea 100644
--- a/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSCatalog.java
+++ b/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSCatalog.java
@@ -81,9 +81,9 @@ public Map<String, String> parseCatalog(String catalogURI) {
             URL url = requireNonNull(FSCatalog.class.getClassLoader().getResource(catalogURI),
                     () -> "Catalog not found in classpath: " + catalogURI);
             return parseCatalog(url);
-        } catch (Exception ex) {
+        } catch (IOException ex) {
             XRLog.xmlEntities(Level.WARNING, "Could not open XML catalog from URI '" + catalogURI + "'", ex);
-            return new HashMap<>();
+            throw new IllegalStateException("Cannot find " + catalogURI + " in classpath", ex);
         }
     }
 
diff --git a/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSEntityResolver.java b/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSEntityResolver.java
index e873b4d78..97c2543ee 100644
--- a/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSEntityResolver.java
+++ b/flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSEntityResolver.java
@@ -56,7 +56,7 @@ public class FSEntityResolver implements EntityResolver2 {
     /**
      * Singleton instance, use {@link #instance()} to retrieve.
      */
-    private static FSEntityResolver instance;
+    private static final FSEntityResolver instance = new FSEntityResolver();
 
     private final Map<String, String> entities = new HashMap<>();
 
@@ -150,10 +150,7 @@ private static InputSource newEmptySource() {
      *
      * @return An instance of .
      */
-    public static synchronized FSEntityResolver instance() {
-        if (instance == null) {
-            instance = new FSEntityResolver();
-        }
+    public static FSEntityResolver instance() {
         return instance;
     }
 
diff --git a/flying-saucer-examples/src/test/java/org/xhtmlrenderer/pdf/ConcurrentPdfGenerationTest.java b/flying-saucer-examples/src/test/java/org/xhtmlrenderer/pdf/ConcurrentPdfGenerationTest.java
new file mode 100644
index 000000000..d2334a1c7
--- /dev/null
+++ b/flying-saucer-examples/src/test/java/org/xhtmlrenderer/pdf/ConcurrentPdfGenerationTest.java
@@ -0,0 +1,92 @@
+package org.xhtmlrenderer.pdf;
+
+import com.codeborne.pdftest.PDF;
+import com.lowagie.text.DocumentException;
+import junit.framework.TestCase;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.w3c.dom.Document;
+import org.xhtmlrenderer.resource.FSEntityResolver;
+import org.xml.sax.SAXException;
+
+import javax.xml.parsers.DocumentBuilder;
+import javax.xml.parsers.DocumentBuilderFactory;
+import javax.xml.parsers.ParserConfigurationException;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.net.URL;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ScheduledExecutorService;
+
+import static com.codeborne.pdftest.assertj.Assertions.assertThat;
+import static java.util.Objects.requireNonNull;
+import static java.util.concurrent.Executors.newFixedThreadPool;
+import static java.util.concurrent.Executors.newScheduledThreadPool;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.SECONDS;
+
+public class ConcurrentPdfGenerationTest extends TestCase {
+    private static final Logger log = LoggerFactory.getLogger(ConcurrentPdfGenerationTest.class);
+    
+    public void testSamplePdf() {
+        byte[] pdf = generatePdf("sample.html");
+        verifyPdf(pdf);
+    }
+
+    public void testConcurrentPdfGeneration() throws InterruptedException {
+        ScheduledExecutorService timer = newScheduledThreadPool(1);
+        timer.scheduleWithFixedDelay(System::gc, 0, 15, MILLISECONDS);
+
+        ExecutorService pool = newFixedThreadPool(20);
+        for (int j = 0; j < 2000; j++) {
+            final int i = j;
+            pool.submit(() -> {
+                try {
+                    byte[] pdf = generatePdf("sample.html");
+                    verifyPdf(pdf);
+                    log.info("Check #{} ok", i);
+                }
+                catch (Throwable e) {
+                    log.error("Check #{} failed: ", i, e);
+                }
+            });
+        }
+
+        pool.shutdown();
+        assert pool.awaitTermination(250, SECONDS) : "Timeout!";
+    }
+
+    private void verifyPdf(byte[] pdfBytes) {
+        PDF pdf = new PDF(pdfBytes);
+        assertThat(pdf).containsText("Bill To:", "John doe", "john.do@mail.com");
+        assertThat(pdf).containsText("Invoice #:", "INV-666");
+        assertThat(pdf).containsText("Invoice Date:", "Sep 27, 2023");
+    }
+
+    private byte[] generatePdf(String htmlPath) {
+        ITextRenderer renderer = new ITextRenderer();
+        renderer.getSharedContext().setMedia("pdf");
+        renderer.getSharedContext().setInteractive(false);
+        renderer.getSharedContext().getTextRenderer().setSmoothingThreshold(0);
+
+        URL htmlUrl = requireNonNull(Thread.currentThread().getContextClassLoader().getResource(htmlPath), () -> "Test resource not found: " + htmlPath);
+        
+        try {
+            DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
+            builder.setEntityResolver(FSEntityResolver.instance());
+
+            Document doc = builder.parse(htmlUrl.openStream());
+            
+            renderer.setDocument(doc, htmlUrl.toString());
+            renderer.layout();
+
+            ByteArrayOutputStream bos = new ByteArrayOutputStream();
+            renderer.createPDF(bos);
+            return bos.toByteArray();
+        }
+        catch (DocumentException | IOException | SAXException | ParserConfigurationException e) {
+            throw new IllegalArgumentException(e);
+        }
+    }
+
+}
diff --git a/flying-saucer-examples/src/test/resources/sample.html b/flying-saucer-examples/src/test/resources/sample.html
new file mode 100644
index 000000000..032397dd2
--- /dev/null
+++ b/flying-saucer-examples/src/test/resources/sample.html
@@ -0,0 +1,561 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+	<meta charset="UTF-8" />
+	<meta http-equiv="Content-Type" content="text/html charset=UTF-8" />
+	<meta name="x-apple-disable-message-reformatting" />
+	<style>
+		.content {
+			width: 595px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			float: none;
+		}
+
+		.content.last {
+			width: 595px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			float: none;
+		}
+
+		.content.middle {
+			width: 595px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			float: none;
+		}
+
+		.logo {
+			margin-top: 30px;
+			margin-left: 30px;
+			left: 0%;
+			right: 0%;
+			top: 0%;
+			bottom: 0%;
+			border-radius: 8px;
+			background-size: 64px 64px;
+			width: 64px;
+			height: 64px;
+			background-repeat: no-repeat;
+		}
+
+		.recipient {
+			width: 260px;
+			height: 64px;
+			left: 310px;
+			position: relative;
+			top: -70px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 16px;
+		}
+
+		.recipient .table .line {
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 16px;
+			text-align: right;
+		}
+
+		.recipient .table .title {
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-weight: bold;
+			font-size: 16px;
+			line-height: 24px;
+			text-align: right;
+		}
+
+		.billTo {
+			position: relative;
+			width: 260px;
+			min-height: 66px;
+			left: 30px;
+			top: -40px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 16px;
+		}
+
+		.billTo .table {
+			text-align: left;
+		}
+
+		.billTo .table .line {
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 16px;
+		}
+
+		.numberAndDate {
+			position: relative;
+			width: 215px;
+			height: 62px;
+			left: 355px;
+			top: -110px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+		}
+
+		.numberAndDate .line {
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 20px;
+			text-align: right;
+		}
+
+		.numberAndDate .line .key {
+			width: 75px;
+			height: 20px;
+			float: left;
+			text-align: left;
+			font-weight: bold;
+		}
+
+		.numberAndDate .line .value {
+			width: 140px;
+			height: 20px;
+			float: right;
+			text-align: right;
+
+			text-overflow: ellipsis;
+			white-space: nowrap;
+			overflow: hidden;
+		}
+
+		.main-content {
+			position: relative;
+			width: 536px;
+			height: auto;
+			min-height: 480px;
+			left: 30px;
+			border: 1px solid #F2F2F2;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+		}
+
+		.information {
+			position: relative;
+			width: 505px;
+			padding: 16px;
+			height: auto;
+			left: 30px;
+			background: #F2F4F9;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 16px;
+			text-align: center;
+			vertical-align: middle;
+		}
+
+		.questions {
+			position: relative;
+			width: 520px;
+			height: 16px;
+			left: 38px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-size: 10px;
+			line-height: 16px;
+			text-align: center;
+			color: #757575;
+		}
+
+		.paging {
+			position: relative;
+			height: 16px;
+			width: 534px;
+			bottom: 0px;
+			font-family: 'SC_ProximaNova-Regular', Sans-Serif;
+			font-style: normal;
+			font-weight: normal;
+			font-size: 10px;
+			line-height: 16px;
+			text-align: center;
+			color: #757575;
+		}
+
+		.charges {
+			width: 536px;
+			height: auto;
+		}
+
+		.charges .header {
+			width: 536px;
+			height: 32px;
+			background-color: #F2F4F9;
+			font-style: normal;
+			font-weight: bold !important;
+			font-size: 11px;
+			line-height: 12px;
+		}
+
+		.charges .header .name {
+			width: 154px;
+			height: 12px;
+			padding-left: 12px;
+			padding-top: 10px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .header .description {
+			width: 205px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .header .quantity {
+			width: 30px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .header .amount {
+			width: 76px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.charges .charge {
+			width: 536px;
+			height: auto;
+			display: table;
+			height: 32px;
+			background-color: #FFFFFF;
+			font-size: 11px;
+			line-height: 12px;
+			color: #000000;
+		}
+
+		.charges .odd-charge {
+			background: #FFFFFF;
+		}
+
+		.charges .even-charge {
+			background: #FAFAFA;
+		}
+
+		.refund {
+			color: #FF3B30 !important;
+		}
+
+		.charges .charge .line .name {
+			width: 154px;
+			padding-left: 12px;
+			padding-top: 10px;
+			padding-bottom: 5px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .charge .line .description {
+			width: 205px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			padding-bottom: 5px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .charge .line .quantity {
+			width: 30px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			padding-bottom: 5px;
+			text-align: left;
+			float: left;
+		}
+
+		.charges .charge .line .amount {
+			width: 76px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			padding-bottom: 5px;
+			text-align: right;
+			float: left;
+		}
+
+		.charges .charge .line .discount {
+			width: 76px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			padding-bottom: 5px;
+			text-align: right;
+			float: left;
+		}
+
+		.charges .charge .line .discount:before {
+			content: "-";
+		}
+
+		.negate:before {
+			content: "-";
+		}
+
+		.resumes {
+			width: 536px;
+			height: auto;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 12px;
+		}
+
+		.resumes .resume {
+			width: 536px;
+			height: auto;
+			height: 30px;
+			background-color: #FFFFFF;
+			font-size: 11px;
+			line-height: 12px;
+			color: #000000;
+		}
+
+		.resumes .resume.bold .key {
+			font-weight: bold !important;
+		}
+
+		.resumes .resume.bold .value {
+			font-weight: bold !important;
+		}
+
+		.resumes .resume .key {
+			width: 408px;
+			height: 12px;
+			padding-left: 12px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.resumes .resume .value {
+			width: 88px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.resumes .resume.blue {
+			background: #D4E9FF;
+			margin-left: 150px;
+			width: 387px;
+			height: 40px;
+			font-size: 11px;
+			line-height: 12px;
+			color: #000000;
+		}
+
+		.resumes .resume.balance {
+			height: 75px;
+		}
+
+		.resumes .resume.green {
+			background: #DBFFD4;
+			margin-left: 195px;
+			width: 342px;
+			height: auto;
+			height: 32px;
+			font-size: 11px;
+			line-height: 12px;
+			color: #000000;
+		}
+
+		.resumes .resume .value {
+			width: 88px;
+			height: 12px;
+			padding-left: 16px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.resumes .resume.blue .key {
+			width: 269px;
+			height: 12px;
+			padding-left: 1px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.resumes .resume.green .key {
+			width: 224px;
+			height: 12px;
+			padding-left: 1px;
+			padding-top: 10px;
+			text-align: right;
+			float: left;
+		}
+
+		.resumes .line {
+			width: 340px;
+			border: 1px solid #D0D0D0;
+			margin-left: 185px;
+			height: 0px;
+			margin-top: 12px;
+			margin-bottom: 12px;
+			margin-right: 10px;
+		}
+
+		.resumes .resume .auto-pay {
+			color: #007AFF;
+			padding-left: 295px;
+			font-style: normal;
+			font-size: 11px;
+			line-height: 12px;
+			letter-spacing: 0.01em;
+			padding-top: 10px;
+		}
+
+		@media print {
+			footer {
+				page-break-after: always;
+			}
+
+			body {
+				-webkit-print-color-adjust: exact;
+			}
+
+			-webkit-print-color-adjust: exact;
+		}
+	</style>
+</head>
+
+<body>
+<div class="content">
+
+	<div>
+		<header style="position:relative; top:70px">
+			<img class="logo"
+				 src="https://img.freepik.com/free-photo/fake-word-yellow-tiles_23-2148227727.jpg" onerror="this.style.display='none'" />
+			<div class="recipient">
+				<div class="table">
+					<div class="line title">
+						<label>title</label>
+					</div>
+					<div class="line">
+						<label>address</label>
+					</div>
+					<div class="line">
+						<label></label>
+						<label></label>
+					</div>
+				</div>
+			</div>
+			<div class="billTo">
+				<div class="table">
+					<div class="line">
+						<b>Bill To:</b>
+					</div>
+					<div class="line">
+						<label></label> <label>John doe</label>
+					</div>
+					<div class="line">
+						<label></label> <label></label> <label></label>
+					</div>
+					<div class="line">
+						<label>john.do@mail.com</label>
+					</div>
+				</div>
+			</div>
+			<div class="numberAndDate">
+				<div class="line">
+					<div class="key">Invoice #: </div>
+					<div class="value" title="INV-666">INV-666</div>
+				</div>
+				<div class="line">
+					<div class="key">Invoice Date: </div>
+					<div class="value">Sep 27, 2023</div>
+				</div>
+				<div class="line">
+					<div class="key">Payment Due:</div>
+					<div class="value">Sep 29, 2023</div>
+				</div>
+
+
+			</div>
+		</header>
+	</div>
+
+	<div class="main-content">
+		<div>
+			<charges>
+				<div class="charges">
+					<div class="header">
+						<div class="name"><b>Name</b></div>
+						<div class="description"><b>Description</b></div>
+						<div class="quantity"><b>Quantity</b></div>
+						<div class="amount"><b>Amount</b></div>
+					</div>
+					<div class="charge odd-charge">
+
+						<div class="line charge">
+							<div class="name"><label>Charge name</label> <label>(boby bryan)</label>
+							</div>
+							<div class="description">Charge description</div>
+							<div class="quantity">1</div>
+							<div class="amount">$666.00</div>
+						</div>
+
+
+					</div>
+				</div>
+			</charges>
+		</div>
+		<div class="resumes">
+			<div class="resumes">
+				<div>
+					<div class="resume bold">
+						<div class="key">Current Invoice Total:</div>
+						<div class="value">$666.00</div>
+					</div>
+					<div class="resume bold blue">
+						<div class="key">Total Amount:</div>
+						<div class="value">$666.00</div>
+					</div>
+				</div>
+
+				<!-- AUTO PAY -->
+
+			</div>
+		</div>
+	</div>
+
+	<div>
+		<footer style="top: 0px; position: relative;">
+			<div class="information">
+				<label>Sed ut perspiciatis unde omnis iste natus error sit voluptatem</label>
+			</div>
+
+			<div class="questions">
+				If you have any questions about this invoice, please contact
+				<a href="mailto:support@mail.com"><label>support@mail.com</label></a>.
+			</div>
+
+			<div class="paging">
+				Page <label>1</label> of <label>1</label>
+			</div>
+		</footer>
+	</div>
+</div>
+</body>
+
+</html>
\ No newline at end of file