LibreOffice for Android and iOS | LibreOffice - Free and private office suite - Based on OpenOffice - Compatible with Microsoft
www.libreoffice.org

Edit: the app seems to be pretty old, I like the Writer editing more (it works) but for other things, CollaboraOffice is way better.

Has anyone tried that on a tablet? The phone UI has like no features

Currently it is a “viewer” (renderer with usable UI) and editing features are experimental.

They only release their APK to Google Play for whatever reason, but you can use F-Droid to get it

Bugs can normally be reported to their bugtracker and you can join their forum for more community support!

Youtube Demo Video

Downside: the editing features are VERY rudimentary, while the app is only about 50MB (⅕) smaller than Collabora-Office, which has better features overall.