If you get something like:
Notice: Undefined index: win-1252 in src/Mpdf.php on line 3741
you might need to suppress NOTICE warnings using e.g.
<?php $former = error_reporting(E_ALL ^ E_NOTICE); // or $former = error_reporting(0);
Include this at the start of your script before instantiating and running mPDF.
It is a good practice to return the former
Also, you could want to report the notice in the mPDF Github issue tracker