Itch is primarily a video game site, but it allows you to upload and sell any type of digital content, such as assets, comics, or books.
The general guidelines are here:
https://itch.io/docs/creators/quality-guidelines
When you create a project, you'll see several main categories, such as: Games, Tools, Game Assets, Comics, Books, Physical Games, Albums & Soundtracks, Game Mods, and Everything Else.
As the creator, you should select the category that best suits your content. For example, if it's a short story in PDF format, it could go in Books.
If the story can be played online and has interactive elements, you could mark it as a game and add the tag "interactive fiction."
https://itch.io/games/tag-interactive-fiction
Keep in mind that the site is related to video games, and users come looking for that. For example, in the Books section, most of the books are related to video games, and it's very rare to find something outside of that theme. Itch doesn't prohibit it, but if you publish a book of poems, chances are none of Itch's users will be interested, but you can still upload and sell it on Itch.