couchtripper

This program is free and seems to be able to read a lot of different file types. It's available for Windows and Mac.

https://www.lexcycle.com/desktop

And here's a Firefox plugin that works similarly.

https://addons.mozilla.org/en-US/firefox/addon/45281

A popular open-source program for converting between eBook formats is Calibre. It works better converting between the more basic formats (epub/mobi/opf) than the more proprietary ones like PDF.