Well, I'm not sure how to do it from the screen, but if you have the keyboard open, you can hit "FN" and then the up or down arrows ("PGUP" or "PGDN").
How funny is it that I was just wondering about this myself not a few hours ago?
EDIT:
This is all I was able to find for page up/down functionality without the hardware keyboard:
http://forum.ppcgeeks.com/showthread.php?t=42212
Post #7 seems to be the way to go, but I don't find the idea of remapping buttons just for this funtion to be particularly sexy.