The Free On-line Dictionary of Computing (19 January 2023):

hlp

    A Microsoft Windows filename
   extension for hypertext WinHelp files.  These are in a
   proprietary format, and are compiled from source files
   written in a dialect of RTF.

   See also gid.

   Usenet newsgroup:
   news:comp.os.ms-windows.programmer.winhelp.

   (1997-01-30)