I also use Skim, it works really well. A pretty cool feature is the back and forward buttons, that hold your page if you skip through large chunks. For example, if I use the table of contents (Cmd + Shift + T) to go to the answers section, I can press back to get back to the question I was on. Then forward brings me to the answers once again.

That's basically my textbook workflow 90% of the time.