This is the source code for the original AT&T version of troff, dated May 1977-1978. It was extracted from a tarball holding the original PWB source published by The Unix Heritage Society:

http://www.tuhs.org/Archive/PDP-11/Distributions/usdl/bostic_pwb.tar.gz

The files in this repository's root directory are garnish added for VCS or GitHub-specific purposes. Everything in the immediate subdirectories has been left untouched from their extraction. The file timestamps date are back to October 25th, 1978.

If you're actually able to compile and run a codebase this ancient, for God's sake, please send a pull-request.