Calibre (which has its own forum here) can convert many DRM-free formats to ePub. This includes PDF, although converting from PDF to reflowable formats like FB2 and ePub is hit and miss. If you have a Windows PC then
Book Designer can convert many formats to FB2 and allows editing of the ebook as well. There are several other FB2 tools, but most of their documentation is in Russian.
The DRM, if any, can be stripped from MOBI (mobidedrm.py), eReader (ereader2html.py) and LIT (ConvertLIT GUI) ebooks, but note that this is generally thought to be illegal in the US and many other places.