The notion of "document security" is a common desire in business and legal contexts. PDFs are a standard representation of documents, whereas JPGs are less common (TIFF might count since it's a somewhat common scanned document format, but I digress).

Subverting the most popular document format, one which even provides built-in support for digital signatures is probably much more interesting than subverting the JPG format.

Open challenge: build two signed PDFs with identical signatures and different contents - it's not necessarily hard, and it would definitely drive the point home.

Because many automated systems/image hosting website (imgur, etc) will process JPEGs to optimize them, which I assume would no longer cause there to be a collision.

On the other hand, PDFs are almost universally sent as-is, increasing the risk of being able to 'fool' someone.

Perhaps to be taken more seriously by management. All things being equal, the content of PDFs (auditor reports) is more important than that of JPGs (cat memes).

Maybe because JPG bytes or more likely to be manipulated when sending around the web?