Undying Dusk might have a credible claim at being the longest choose-your-own-adventure book ever written, cramming a fully-featured dungeon crawler into a hefty 200,000 page PDF file.
Available for free over on Itch.io (opens in new tab), Lucas Cimon's retro RPG doesn't immediately jump off the page—after all, there are dozens of classic dungeon crawlers on the store. That is, until you realise the game is effectively one giant book, each page a single frame of its sprawling labyrinth.
It works in the same way as any old paperback adventure book. But instead of explicitly saying "turn to page 56 to slay a goblin", each page has a faux-UI full of directions, actions, and menu buttons that link to other parts of the document. There are even musical cues, with soundtrack buttons that'll open your web browser to play some fantasy tunes.
Cimon claims that, to the best of his knowledge, this is "the very first video game in a PDF format". That might not be entirely true (a cursory search shows some simple noughts-and-crosses (opens in new tab) games and a now-broken maze (opens in new tab)), but Undying Dusk is certainly the most elaborate—a fully realised fantasy world featuring 10 locations, an overworld, and a cutscene-driven plot. In keeping with the spirit of old adventure games, it's also pretty brutal, sporting thousands of game over screens and only a single path to victory.
Undying Dusk is free to download. Cimon recommends playing it on Sumatra, though it should be playable on pretty much any PDF reading software (except Adobe Reader, for technical reasons).