Welcome to PDOS (Public Domain Operating System)
There are two major components to PDOS.
First of all there is the operating system itself, which is designed to be a clone of MSDOS, except that it comes in both 16-bit and 32-bit vesions.
Secondly there is PDPCLIB, a public domain C runtime library. PDPCLIB works on DOS, OS/2, Win32, PDOS, MVS (mainframe) and CMS.
You can find documentation for PDOS and PDPCLIB in text files in the downloadables here.