Why does Word tell me a document has changed when it hasn't?

The most common cause happens when you have receive a document from someone else, and you have "background repagination" set. If your printer is slightly different than the printer the originator used, then the first time you look at a document it will be reformatted to fit your printer, resulting in what Word considers a change.

Another common cause is something called "fields", which are in essence calculated results. For example, you can insert a field into your document that includes the last date that the document was saved or printed. The problem arises when fields are recalculated just by looking. Fields including date and time are often culprits, though in complex documents there are several approaches that could result in Word thinking the document has changed. It's difficult to say exactly which, because that will vary based on the document in question.

Finally Macros and Add-Ins can also cause Word to think something has changed. Macros can be set to trigger when a document is opened or at other seemingly random times. Add-Ins similarly can make subtle changes that may not affect the document visibly, but can cause Word to think it's changed.

The long, drawn-out, bottom line is that it can often be hard to tell why Word might consider a document to be changed even though you haven't done anything. Hopefully I've provided a few clues.