Firefox 7 and Chrome 13 let you prefix just about any URL with “view-source:” and see the raw bytes. It works for GIFs, PDFs and other binary assets.
Since PDF is a quasi-ASCII format, you can easily find interesting details like this embedded in the markup:
It doesn’t work in Safari 5.0 or IE 9 — they don’t recognize the “view-source:” prefix. Not sure about other browser versions…