PDF to Epub
You can convert a PDF into an eReader-friendly size PDF by using this software:
k2pdfopt, from willus.com
This is a strange piece of software.
Once you've converted a PDF into an eReader-friendly size, you can then conver it to ePub using Calibre software.
Another option, if you have MS Word, is to re-flow the content in MS Word, and then convert the resulting file into your desired format (again using Calibre). More is written about this technique here: https://www.willus.com/k2pdfopt/pdf_conversion.shtml
...at that page Willus says:
MOST RECOMMENDED SOLUTION (Updated 28 May 2017)
While I prefer open-source solutions, if you already have a recent version of MS Office, try loading your PDF directly into MS Word. MS Word now converts PDF files to Word documents amazingly well (another example here). Microsoft refers to this as PDF Reflow, and it has been available since MS Word 2013. Word's PDF Reflow even automatically finds the text in scanned/bitmapped PDFs and uses (quite accurate) optical character recognition (OCR) to convert it to editable text characters in the Word document. Once you have your PDF file in MS Word format, you'll have a lot more capability to manipulate it into other formats and/or form factors. For example, you can use the free e-book management utility, calibre, to convert .docx to epub/mobi, or you could use Writer2ePub, ePub Tools--a Word add-in, or pandoc (cross platform).