The PDF creating application needs to ensure that the document is accessible. Adobe Reader (on desktop) does support reflow [0]. Adobe Reader on Android also supports reflow [1].

[0] https://helpx.adobe.com/acrobat/using/reading-pdfs-reflow-ac...

[1] https://www.quora.com/Whats-the-best-free-PDF-reader-on-Andr...

I use this tool to reflow PDF papers for an e-book reader: http://willus.com/k2pdfopt/