Office Mobile not installed yet opening PDF files

I have a Samsung A50 phone, stock Android which came with the device (from Comcast), everything up to date and not rooted. I have not registered for the Samsung Galaxy store or whatever is called, and therefore never installed anything from it. I never installed Office Mobile on my phone, and looking at the list of installed apps in Google Play shows no Office Mobile. Yet, starting today, attempting to open some (but not all!) PDF files (by tapping their name in the Document folder) opens them in Office Mobile rather than the "Google Drive PDF viewer" where they did open till the day before yesterday (I did not try yesterday). There is not an option to force opening the files in "Google Drive PDF viewer". For a test, I installed the "Google PDF Viewer" (I wish they chose less similar and less confusing names). After that Android allows me to open the files with "Google PDF Viewer", but it fails with "Cannot display PDF (no file received)". After a reboot, the files were gone. What may have happened? I can guess that the files corrupted and the "Google Drive PDF viewer" magically knew (and therefore was not proposing itself to open them), whereas the other two apps did. Yet, why is there an app which I never installed and does not show up in the list of installed apps? Heck, if I search for it in Google Play, I even find it, and it shows me the option to install it!