Sadly PDFs are usually just a series of graphics instructions for drawing stuff (including text) at specific sizes and positions, on a page with a defined size. It's an inherently hostile format for reflowing text, although I think some PDF viewers can do that to an extent. The PDF accessibility spec defines an optional, more DOM-like document structure that a compliant viewer could use to reflow text, but in my experience even the apps that produce that structure don't populate it with enough data to make reflowing reliable or easy to implement. Pretty much every format is way better for reflowing text than PDF (Kindle, ePub, HTML, etc.).