Can't find any references to that except that each doc has a GUID. A GUID on its own is not PII - just some random number, so are you implying that MS collects every GUID along with author identifiable information?

I just got off the phone with a lawyer to talk about this exact issue.

If the GUID is related to the user (like user ID), then it is Personal Information - EVEN if the GUID is random. The distinction that is easy to miss is that a User ID GUID might be very low risk (compared to, say actual User Id or user name) - but is is still Personal Information.

If the GUID is for the document (and anyone can edit the document), then it is no longer PI.

Of course, all of this ignores things like the contents of the doc. If the doc is "SSNs of my customers", well... don't do that