Format differences for offline adult reading aren't just about file types—they're shaped by what you're trying to do with the book. I mostly read on a Kindle Paperwhite, so the MOBI or AZW3 files are my usual go-to for downloaded novels. The key is making sure the formatting converts cleanly, especially for complex nonfiction with footnotes or diagrams; a PDF might hold the layout better, but it's terrible for adjusting text size on an e-ink screen.
For audiobooks, the offline format battle is between proprietary app downloads (like Audible's AAX) and DRM-free MP3s. I subscribe to Audible, so my library is full of titles I've downloaded through their app for commutes without service. It's convenient, but it locks me into their ecosystem. I've started buying some DRM-free MP3 bundles from authors' websites for stuff I want to keep accessible long-term, even if I cancel a subscription.
The real shift I've noticed is in serial fiction platforms. Apps like Radish or WebNovel let you 'unlock' chapters with daily passes or coins, which you can then download for offline reading. It's a weird hybrid model—you 'own' the chapter for offline use only as long as the app and your account are active. It feels more like a long-term rental than a permanent download, which changes how I think about building a personal library.