FWIW: I haven't yet met a PDF file that would be unflowable. The results vary of course, reflow does horrors to things like python code
PRS-650 has one more convenient thing: page mode, that allows you to zoom in to a fixed size, and then use page buttons to move the zoomed in area by pages. There are modes for 2 and 3 columns text (useful for science papers and rpg manuals). I'm not sure whether K3 has this kind of feature.