SumatraPDF is a multi-format (PDF, EPUB, MOBI, FB2, CHM, XPS, DjVu) reader
for Windows under (A)GPLv3 license, with some code under BSD license (see
AUTHORS).

Want to join the project and contribute code? Read
https://github.com/sumatrapdfreader/sumatrapdf/wiki/Contribute-code

To compile you need Visual Studio 2013 (free Community edition from
http://www.visualstudio.com/en-us/news/vs2013-community-vs.aspx will work).

Use vs/sumatrapdf-vc2013.sln.