On mobile devices, text markup languages are the way to go. Viewing office suite documents is enough… editing them is not easy. I have used groff, tex and latex, and typst recently (on Termux, obviously) with various degrees of success. I wonder when we will have FOSS apps that are as usable or more than Google’s Docs and Sheets.

If your PDF viewer supports re-reading when file is changed (or reloading file every time app comes to foreground, I suppose), typst watch and that viewer open is the closest to a WYSIWYG I could get working.