Go to /Users/your-user-name-here/Library/Group Containers/UBF8T346G9.Office/User Content/Startup/Word
Restart Word and problem solved
Fix similar problems in PowerPoint and Excel:
Remove the SaveAsAdobePDF.ppam and SaveAsAdobePDF.xlam from the PowerPoint and Excel folders next to the Word folder.
Launch Excel and PowerPoint and go to the menu Tools -> Excel Add-ins... and Tools -> PowerPoint Add-ins..., respectively, and remove the Save as Adobe PDF add-in, by un-ckecking it and removing it with the "-" button and then click OK.
Restart Excel and PowerPoint and the problem should go away. If not, try again.