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.