[DSpider]

First, you need to figure out how to extract the text, with all the bolds and italics (don't worry about the chapter titles, footnotes, etc). Copy-pasting won't work. You may have to run the PDF through ABBYY FineReader and OCR it. Then you need to recreate the layout, either in Adobe InDesign, Microsoft Word, or whatever.

Adobe InDesign will export it as ePub. Recreating it in Microsoft Word will also work, but then you should export as Filtered HTML and import that into Sigil. There's also the fact that FineReader itself can export as ePub (FineReader 11). Don't forget to proofread it at least once. Tracking down the fonts or matching them with something that looks similar is optional, but adds a nice touch. You can find the name of the fonts if you open up the PDF in Adobe Reader and go to File - Properties - Fonts.