I would prefer it to be open source, but at this point that’s not a necessity. I need an app that can open word documents, powerpoint presentations and excel spreadsheets (maybe PDFs as well but I could always get a separate app). So far I’ve tried Openoffice & Libreoffice Document reader from F-droid which worked fine for documents but simply would not load pptx or xlsx files. I’ve now installed WPS Office from the Play Store but all the conversion options are clogging up the share with… options every time I want to send a picture. I would appreciate any recommendations at this point.
WPS, which you have already installed, is unfortunately going to be your best option, with internet turned off. Collabora (LibreOffice mobile fork) has BAD decoding and compatibility issues with the MS Office formats, which are a staple of how documents are shared around.
I just use an older WPS version 12.7 with no internet, and my share menu is not clogged either. Librera Pro is not an option since it only views DOCX, not edits or even fulfills your use case for PPTX and XLSX.