Until Calibre gets its new PDF engine, you might like to take a look at a new utility called
"pdfreflow" posted over in the PDF forum.
https://www.mobileread.com/forums/showthread.php?t=83094
It converts your PDF to HTML and reflows the paragraphs as best it can from the info you input. You would then import the HTML to calibre for conversion to EPUB.
I have been pleasantly surprised with the results I've had so far, although it's not guaranteed to work with all PDFs.