Issue when using with Laravel 6.*.*

[ElMorin89]

There is in issue with Laravel 6 and over. The str_random() is not available anymore into laravel. We must add the following into PDFMerger.php
use Str;
and change str_random(16) to Str::random(16)

We also need to add Laravel\Helpers to composer.
https://laravel.com/docs/6.x/upgrade#helpers