I'm going to push back on your question a bit -- why would you want to convert from PDF to epub?
Both are rendered formats, and their rendering is specific to the tool used to render them. PDF's by their very nature are made in order to be printed according to a specific dimension.
I'm guessing that you want to do backend processing for a website or business that has legacy PDFs. Perhaps you want to redistribute the files to a more mobile-friendly format?
Now that tablets of 9 inch and higher are popular, it is fairly easy to use a PDF viewer app to read almost any pdf tolerably well. Why not just treat PDF as an end format for viewing -- and not worry about imperfect conversions?
A person with an android could potentially upload all these pdfs to Google Play Books and have a high quality way to read all the pdfs.
I am guessing that you have already googled for batch conversion from pdf to epub. (see also this: https://www.quora.com/What-is-the-best-free-PDF-to-EPUB-converter ) (Flawed) Solutions certainly exist, but I am having trouble understanding the business need.