Thanks, Lisa, I understand now. I'm used to reading PDF files in a 4" Pocket PC, and that's how I read two-column books on that device using Acrobat Reader when reflow is not available on the file or is inconvenient, first setting the zoom level without reflow and then moving across the pages from top to bottom, left to right. But now I see that the Reader provides a better solution with its reflow capability, in spite of the fact that the images may be lost with the reflow.
Also, I found a video that explains the PRS-600's zoom function. This video shows that one can pan in every direction in zoom mode using movements of the finger as well as using the panning arrows.