PS: If, despite the above warning, you decide to go ahead, I recommend not to use Calibre for PDF conversion. I've had better results using the old
Mobipocket Reader (killed by Amazon similarly to Stanza). Here is the
install file for Windows. The result of the automatic, one-click conversion will still be very poor, but that's just the way it is.
For optimal quality of conversion from PDF to EPUB, you need to sacrifice all those extra hours of manual work, and use top-quality OCR software such as FineReader. I create a HTML file in FineReader, then fine-tune that HTML file by manual coding (using the EditPlus plain-text editor for Windows) until its code is approved by W3C's validator. No fluff must be left inside the file -- the CSS must be minimalistic. Finally, I convert the HTML file to EPUB in Calibre, and that's it.