PDFs: not quite unlinkable
“Apparently, just like with named anchors in HTML, although you can only go to a specific spot if the original author put in named destinations, you can still do quite a bit with links to PDFs.”
Also, don’t miss Why PDF sucks. The biggest issue is that PDFs are proprietary. Or perhaps more accurately, PDFs are a proprietary format (i.e., Adobe controls the specification).
While I’m talking about PDFs, I might as well mention that you can freely create PDFs from almost any Windows document using CutePDF. That means you can avoid buying Acrobat in many cases.