Maybe you don't need to convert pdfs to epub, but just optimize pdf for 6" reader using k2pdfopt, if layout is not too complicated.
http://www.willus.com/k2pdfopt/
Once optimized it is not a big deal to convert it to epub.
You can also ask for some help in author's thread, after you learn the basics of optimization with k2pdfopt.
https://www.mobileread.com/forums/sho...d.php?t=144711
There is PdfMasher, but I have never used it.
http://www.hardcoded.net/pdfmasher/