alt Hacker NewsI'd say the reason for this is that PDFs are not viral like images or videos. I don't think any of the technical arguments are strong enough.
If you want to forge a PDF, you do it because you have a private target in mind. Financial fraud, corporate espionage... There are other settings but people don't just share PDFs around for you to see a deluge of fake ones.
I believe that some of the DNC (and other hack & leak) documents were tampered with. Does anyone remember if that included pdfs?