Design document, from the page that links this pdf (http://gameshelf.jmac.org/2008/11/grim-fandango-puzzle-desig...):

"Tim Schafer at Double Fine has posted the puzzle design spec for his classic adventure game, Grim Fandango. It is that game's tenth anniversary; it was released on the Day of the Dead, 1998."

Looks like Tom Schafer style; I find him whimsical and funny in all of his communication. I don't think it's a stretch that this aspect extends to nom-public communications.

It's a game design document.

My guess is it is kinda like a first draft that some higher ups approve and budgets and then dialogue and game development and puzzle testing can begin.

Looks like a GDD but just for the puzzles, a PDD