The main problem here is that your intended document was not designed to be displayed in small screens in the first place. If it was, then it should support reflow-able text.

To take this issue to the extreme, imagine trying to read a plain 'ol newspaper rendered as a PDF. You can ask for a 2560x2048 3.5" screen (or whatever) to be able to render the whole width of the content but you'll still end up finding the bottleneck elsewhere (your eyes).