It's "groff Collected Reference Pages": a 402 page document, in PDF #format, https://www.gnu.org/software/groff/manual/groff-man-pages.pdf
It was made by G. Branden Robinson: thanks to him & congratulations for this work!
He announced it on groff open mailing list.
And moreover, you can navigate throught the pages: there are PDF hyperlinks!
And of course, the colophon is "made with groff"
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