The Surface was factory reset only a few days back. I don't use it that much except for occasional ebooks. The only software I had in it were Firefox, Chrome, Office and Drawboard if you don't consider all that Candy crush/Soda crush/Animal kingdom bloat that MS likes to push on us (which I promptly uninstalled).

I have to admit that I downloaded the pdf ebooks from piracy sites. So don't know if they had some malware in them. I did scan them with MBAM, Avira, MS Defender before use though. Note that i didn't download them from Surface. I downloaded them using a Ubuntu VirtulaBox VM running on another laptop. I restore the VM to a previous snapshot each time after use.

It could very well be the original Microsoft software doing this. Check if your Microsoft account and other cloud accounts you are using on that machine have been compromised.