I’m building a fanfiction archive and I was wondering whether I should add federation just to the forum or to the whole site and if I add it to the whole site whether or not ActivityPub would be a good fit, like is it possible using ActivityPub to share the load of hosting the files across different instances or is that something unrelated and it’s only useful for typical social interactions?
The clue of what ActivityPub is for is in the name: it is for publishing activities.
Some such activities could be that someone has written, updated, or commented on fanfiction stories. If you want to exchange that information between different sites, AP is very well suited for that.