Since I figured it out: for anyone who'd like to know what you're meant to do... when you submit the papers, if you got it right you'll progress to the next step, and if you got it wrong it reappears again as a blank. You'll have to fight the buggy controls very much... LMB selects a random object to pick up rather than the one on top, and this also randomly sets which document will be stamped, choosing a randomly different one each time you pick it up again.
(short guide: if holding a stamp and right-clicking won't stamp it, pick up the document then try again until it eventually selects it as the right one & allows stamping it). It's also possible to accidentally stamp the edge of a different document by 1 pixel and not realize it.
Maybe there's a better way? But I'm not quite sure what the description meant.
Very short, simple, and without the nice stuff a game has. But a really great idea for the jam and of the few games I've played the idea is the most novel so I love it for that, even if it suffers really really badly from the bugs, no tutorial, and so on.