I think this might also have affected my Win7 machine. I use Microsoft Office online version and this morning all my pdf file icons were different and files were missing. Not enough time this morning to sort it out though.
That's pretty scary, but I don't see offhand how that could be caused by a Windows 10 update. Maybe something changed in their cloud ("online") storage as well, though. Please post back an update to this comment when you have it figured out -- that's one I'd want to watch!
May be re-associated to Edga instead of Reader.
You can change it back easily if so.
They’ve been pushing using Edge for PDFs for a while.
They love Edgs, I expect IE will be gone soon.