Documents that are downloaded to PDF open in word and are unreadable.
Select the icon of a file you know is a PDF. Right click on it, then Properties > Open with, click on Change. Choose Reader, and check the Always open with option (words may vary a little).
I wonder why this happens so often recently; every day there are at least a couple of posts with the same problem.
To the o/p: when did this start to happen? Was there any software update around the time when it started?