Before the new '.docx`, '.xslx', etc. formats, when it was still just .doc, .xsl, etc., the document format was (as I've heard it told) essentially just a memory dump of Word/Excel's state for that document at the time you saved. And since it's easy to imagine that serializing/deserializing such a complex thing might not be always 100% perfectly idempotent, it would indeed happen that just the act of opening a file would change it in some subtle way.

That hasn't been the case in a long time, though.