Quick summary: Most PDFs on the web should not be PDFs. PDFs can be made accessible, but you need to think about document structure and semantics, just like with HTML.
If you ask me, the most irritating things with PDFs are these:
- when they open in a new window
- when a plugin that takes half an hour to load displays them in my current tab/browser window
I want them out of my browser and into an application like Preview.