First, for those who apparently don't know already, Sigil is an editor which allows you to view and edit the individual HTML files, meta data, toc, etc. No need to unzip an ePub as Sigil already has that covered. Sigil lets you open an ePub and gives you instant, direct editing access to all the source files zipped up in the ePub.

Secondly, I agree with JSWolf. PDF files are the most gawd awful files in the publishing world. If you are trying to convert PDF to ePub then expect daily migraines followed by eventual brain shut down. If you have the original source files used to create the PDFs then use them for the conversion. If you don't have the original source used to create the PDFs then they were probably not created by you, so you might not have a choice but to use the PDFs, but that will be a grueling task.