PDF's are not specifically a native filter option in DFP2.
You could restrict files other than PDFs with client side validation, or you could play with the actual extension file.... HelperPHP.php
This is just a test suggestion, and YOU SHOULD MAKE BACKUP COPIES before you even think about doing this.
IF you felt up to playing with the code... You could try changing ALL of the PNG references to PDF. It might be a good idea to follow the CASE sensitive flow that is already in place in that file. There are CAPITAL PNGs and lowercase png references in there.
No promises, but it might work pretty well...