Hello,
I found the problem and I was able to fix it on my installation.
The problem was due to a corrupted library, somehow the code has been modified and it caused the MPDF library to break.
Developers: Take a look at application/helpers/mpdf/mpdf.php on line 6361, that shouldn’t be that way? It is broken code. Someone pasted it poorly or something. This will be broken for anyone who downloads the latest version from the website, just as it was broken for me.
Users Looking For A Fix: Download the original MPDF library files from this URL: (http://mpdf1.com/repos/MPDF574UPGRADE573.zip), extract the Zip file, and copy the source “mpdf.php” to your application/helpers/mpdf/ folder, overwriting the file which is in there.
This corrected the problem for me 100%. 