Is there at least some text indicating that the mPDF script is still working on creating a PDF file?
I tried to make a simple Echo before running the script, but it looks like it is not showing, or mPDF clears it or something like that.
The solution is to turn on mPDF progress indicators! inside /mpdf/config.php
/mpdf/config.php
$this->progressBar = 1;// Shows progress-bars whilst generating file 0 off, 1 simple, 2 advanced