Skim is a free, open-source PDF viewer for Mac OS X. Mac OS X comes with Preview, which views PDF files, and Adobe Acrobat Reader is freely available so what makes Skim special? Skim has features that make reading technical articles and academic papers easier.
The first of these features is the ability to add notes to a PDF document. It's the electronic equivalent of adding sticky notes to a paper document. When you're reading difficult material, being able to make notes to yourself helps in understanding the material.
A second useful feature is the ability to highlight areas of text by putting a circle or box around it. A third feature is snapshots, which let you keep important pieces of a document in easy reach.
Skim is at version 0.2, which means it's far from a finished product. But if you read a lot of PDF technical articles, you should give Skim a test drive.