Webnotes, a service that lets you highlight and add floating sticky notes on top of live Web pages, now has a pro version. For $9.99 a month, users get the option to mark up not just normal Web pages, but PDFs too.
Some competing annotation services like Diigo and SharedCopy do not offer the capability to make annotations or leave highlights on PDFs, so this is a big deal for students and business users who are likely to run into them frequently while doing research.
If you come across a PDF you want to mark up, Webnotes can convert it into a special Flash-based PDF viewer that's got the Webnotes mark-up tools built in (you can try it here). It also saves the entire document to your Webnotes account so you can access it even if the page goes offline.
Other premium-only features include support in case something goes wrong, and the option to highlight content in multiple colors. You can use this feature to sort your annotations within any folders by color. This makes it easier to organize notes after you've taken them. Back when I was in college, I used to do this with about $15 worth of color-coded Post-it sticky flags when digging into research on big papers. So if you're using Webnotes as a sidekick to your book research, you can make use of this system for a unified organizational approach.