Quick dirty script you can use in a job if you need to merge PDF's together.

merge-pdf.php

<?php
namespace PDFMerge;

use Symfony\Component\Process\Process;
use Symfony\Component\Process\Exception\ProcessFailedException;

// new empty array to fill with appendix documents.
$doc_array = [];
// add to the document array for each appendix item.
foreach($documents as $doc) {
    $doc_array[] = storage_path() . '/app/' . $doc->filepath;
}
// in case anyone else get's stuck :)
$filename = str_random(20);
$path = storage_path() . '/' . $filename
// depending on what workstation i'm working on, switch between the windows cmd
// or the unix cmd, either works.
$unix_cmd = 'gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile='.$path.' '.$implode;
$win_cmd = 'gswin64c -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile='.$path.' '.$implode;

// install ghostscript on the vps for this to work. :(
$process = new Process($unix_cmd);
$process->run();