How do I view documents on my Synology NAS via an Android client instead of downloading them.

I have a Synology as my home server. My dad has a bunch of documents that he uploads to the Synology and then views them, as of now, using DS file (Synology's app for File Station) on Android.

This "works" but every time he needs to view something, the app downloads it and he needs to use a viewer on his phone to then open the document. This is a whole bunch of duplication and clutter I'd like to get rid of.

I have both Synology office and PDF viewer installed but have not found a way for the Android app to use them instead of downloading.

I then moved on to try using the Android app for Synology Drive to view documents instead and it too does the same thing where it downloads the file instead and then uses a viewer on my phone.

I then tried using my Paperless for this, even though I'd rather not and well, their app does the same thing.

Next up on my list is libre office but before I went banging my head at this any more, I just thought I'd post here to see if anyone has clues for me. Something like Plex but for documents. I want to "Stream" my docs, mostly excel and PDF files, from my NAS to an android device (not via the browser). Why has that proven so unreasonable? Help, I'm frustrated. Thanks.