In addition to the Page Up and Page Down buttons, you can scroll a page at a time in Acrobat Reader with Shift Up Arrow and Shift Down Arrow. This might be a known "feature," but I can't see these shortcuts mentioned in the menus.
[robg adds: Why bother, you might ask, given that you have page up and down keys? If you're using the arrow keys to scroll a line at a time, it's much easier to press Shift and an arrow to jump a page than it is to move your hand up to the paging keys...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030305123651791