Maybe this is common knowledge by now, but newer OS versions of Preview are inching toward more & more functions, beyond a quick look at a file. It has even assimilated a few of the most basic Acrobat functions. You still need Acrobat for sophisticated work, but these can be helpful if you don't have or need Acrobat normally.
Re Preview 4.1 OS 10.5.4:
1. Annotation: In the menu "Tools," go to "Mark Up" and "Annotate." The available functions are simple, but nice to have occasionally. I didn't try it, but I think "Mark Up" probably works only with a file that has "readable" (OCR) text.
2. LOVE this one!: It is possible to combine pdf's. Open the different files in Preview (NOT in Reader). Here's the trick: Make the Sidebar visible. It has thumbnails of the pages in the document. THEN, drag the thumbnail(s) to the sidebars to create the combination desired.
After combining the documents, the thumbnails can be dragged into the desired sequence.
BEAUTIFUL! - And finish, of course, by saving the file.
"For what is age but youth's full bloom,
A riper, more transcendent youth" - Oliver Wendell Holmes