Good way to convert pdfs to epubs on the mac?

Most of these are scanned pdfs.

Some pdfs can freeze Preview, Skim, or either e-reader. And generally pdfs are much harder on the Kindle Dx than mobis are.

Some of these have text layers, some don't. Extracting the text layers, removing the line-braking hyphens, and resolving the misspellings could help.