First of all, I love my 505. I bought it for clarity - read for hours at a time and no perceivable eyestrain.
With DDHarriman's suggestion, I just cropped the margins off a pdf with lots of figures and charts - ended up about 6x8. Liken it about 9 point printed size. Much better than before.
If your pdfs are mostly just text, and you don't care about paragraph full-justification, zooming will be just fine. Otherwise I bet an epub or lrf conversion would end up better.
Although it works very well for "real" ebook formats like epub and lrf, zoom is not _my_ favorite feature for pdfs, since (for editables) it removes the graphics and full-justification formatting.
If they ever implemented a centered-percentage zoom in PDFs, I'd be much happier.
Landscape mode works really well for "just a little too small for portrait" situations. it keeps original formatting, including graphics and layout.