On my Palm the equivalent 5-way buttons are handled much more intelligently. The "down" key will try to do the Right Thing, which is mostly work as a next-page key but if you start some other line-by-line or link-by-link operation it'll change to that. One page scroll on the scrollbar restores the next-page functionality.
As it is now I use the stylus way more that I want to, and I haven't yet found a way of changing pages in the PDF reader without de-selecting full-screen mode.