When I asked my original question, I was advised to ask this part of it as a separate question.
I regularly write reports for my employer, and email them through. I write the reports in Word 2016 for Mac, and my employer reads them in Word 2010 for Windows.
Recently, my employer has had the following banner appear whenever he opens one of my reports:
I've done some testing on this, and it appears that this banner is displayed whenever my employer is opening a .doc file from me that I have created in Word 2016 for Mac. .docx files created in Word 2016 for Mac don't have this problem; neither .doc files or .docx files created in Word 2010 for Windows have this problem.
Any idea what the issue is here? It does seem that .doc files created in Word 2016 for Mac are around ten times bigger than .doc files created in Word 2010 for Windows, but I haven't been able to spot anything else that seems relevant.