It's meant for momentary zoom, as you put it. When in zoom mode (as opposed to using the font size changer) you can only pan around the current page using the on-screen arrows. If you wish to go to the next page you'll either need to close zoom mode first, or use the hardware page turn buttons which will put you back in normal (non-zoom) mode and advance (or move back) a page.
For continuous reading of PDF text, the font size change function would make more sense unless you needed the zoom function to better see an illustration.
-------------------- Lisa Gade Editor in Chief, MobileTechReview