Unlike other readers, Skim is a program with a well developed interface, totally open source and that student's will probably like because it allows them to add notes and bookmarks to PDFs.
Thanks to Skim it is also possible to underline the text and it even supports the use of the Apple Remote to change the page. Something very useful if we want to show a PDF to a class with a projector. It is also compatible with LaTeX and PDFSync.
With Skim it is possible to save the pages that we have liked most of a file in favorites, it integrates into Spotlight and it even allows us to save passwords for encrypted files. To sum up, if you want a PDF viewer and you don't like the one provided by Adobe, Skim is one of the most interesting options.