Nitro PDF Reader is a powerful, free Adobe Reader alternative
There are all sorts of compelling reasons to try an alternative PDF reader, security not being the least. Adobe Reader is also quite stingy with its functionality – you can't even annotate PDFs.
Nitro PDF Reader is an alternative reader with a modern-looking interface, and it offers generous annotation options. Unlike Foxit Reader, Nitro doesn't appear to watermark your PDF when you annotate it. You can highlight sections of the document, add text, add sticky-notes, and stamp your signature (not a digital signature – just a scanned one).
If you're lucky, you may also be able to extract graphics and text from your PDF. Ironically, I was unable to extract the images from Nitro's own sample PDF (the Welcome file that comes with the reader). Another thing you should know is that it's quite a memory hog. I tried the same document with both Foxit and Nitro; Foxit clocked in at 8MB, while Nitro consumed a whopping 63MB. Again, this is for the same exact one-page document. This is even more than Acrobat Reader 9.3, which consumed 50MB for that same document.
Still, if you've got a little RAM to spare (and most of us do), I think Nitro is a worthy contender, if only for its annotation capabilities and simple interface.