I cannot find fitz.c file in mupdf folders. I can find fitz.h but, for example, I am not able to inspect the function
void fz_debug_obj(fz_obj *obj);
in detail, and other things I need. Is it possible that some proprietary code is there so that using mupdf is not legal and they sue you when you make a derived work?
mupdf's full source is available via git at git://git.ghostscript.com/mupdf.git or in tar.gz format http://mupdf.googlecode.com/files/mupdf-0.9-source.tar.gz via http://code.google.com/p/mupdf/downloads/list.