If the PDF contains a text layer, i.e. if you can search it, you could try converting it with
Calibre with all Heuristics Processing options enabled.
Activating all Heuristics Processing options often leads to somewhat better results. However, as
Tex2002ans has already pointed out, PDF files are the worst input format for ePub converstion.
Here's a screen shot of the dialog from SoftPedia: