To make LibreOffice work on Mobile is an effort that has already been undertaken by Collabora, who offer their LIbreOffice-based Collabora Office for Android and iOS, the source code is at GitHub - CollaboraOnline/online: Collabora Online is a collaborative online office suite based on LibreOffice technology. This is also the source for the Collabora Office apps for iOS and Android.. It works okay on Android and iOS in my limited testing.
Given that their “LibreOffice on the Web” thing lives in the same repo, I have some hope, that the mobile implementation is basically “the web thing + a browser runtime” — but I have not put any work into finding out if that is actually true; I will try with the Nextcloud instance of a friend when I am back and home without intermittent internet. If the hypthesis turns out to be true, one could try and make this work on Mobile Linux/the Librem 5 with some simple Web-runtime like Cog (or other webkit2gtk) and maybe a docker container (or similar) to locally run “web libre office”.
That’s the way, if editing is necessary. If it’s not needed, and viewing a document is enough, I recommend Install Morphosis on Linux | Flathub and the Document Viewer app (Evince, or, going forward, Papers).
BTW: Back in the day (ca. 2008, in its GTK2 days) Abiword had a mobile interface, but when I tried to build it a few years ago the option still existed but would not compile anymore.