Its a text file, open it and have a look. Possibly it is the file mentioned here: ww1.microchip.com/downloads/jp/DeviceDoc/jp569691.pdf but it could be anything.
Header (.h) files are not mandatory, they exist to simplify programs, especially where several source files share the same declarations. The contents of a header file are passed verbatim to the compiler so it just saves the programmer typing it into each source file individually. Maybe your programmer has rearranged to code to collect common declarations together into one file.
Brian.