PDF Annotation on Mac: Forget PDFPen, use Preview!
If you are looking to annotate PDF files on your Mac, you may not need to download anything. Apple's own Preview software should get the job done; look on the right side of the toolbar (you may need to show the toolbar by clicking the button at the very top-right of the window).
The original version of this post advised readers to skip PDF Pen completely, because it added little value over Preview.app, because it was really buggy and crashy, and because the free version added huge watermarks.
But that was back in early 2007. I’m sure the software quality has improved a good bit since then.
And it really can do things Preview can't. Here are two big ones:
You can edit text in PDFs (not just annotating/adding)
You can remove sensitive information completely. (Drawing black boxes over the information isn't enough, as software -- like PDF Pen, ironically -- can remove those boxes again later. This is because the way PDFs work internally, which is very different from image formats like PNG and JPEG.)
So what am I saying?
See if Preview.app's annotation tools meet your needs.
If not, check out PDF Pen.