How to fix Run-time error 53 every time I open a document in Word or a worksheet in Excel on Big Sur?
After looking really hard for a solution, I finally found in a very obscure page the answer, which is quite simple and works with the latest Microsoft Word on Big Sur:

Close all Office applications
Go to /Users/your-user-name-here/Library/Group Containers/UBF8T346G9.Office/User Content/Startup/Word
Remove linkCreation.dotm
Restart Word and problem solved

Unfortunately that does not solve the problem for PowerPoint or Excel because if I remove SaveAsAdobePDF.ppam inside the PowerPoint folder or the SaveAsAdobePDF.xlam inside the Excel folder will trigger another error on application launch.
To fix these errors, you need to:

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 -&gt; Excel Add-ins... and Tools -&gt; PowerPoint Add-ins..., respectively, and remove the Save as Adobe PDF add-in, by unckecking it and removing it with the &quot;-&quot; button and then click OK.
Restart Excel and PowerPoint and the problem should go away. If not, try several times to remove those options from the Tools menu until the problem goes away. I had to repeat this procedure in PowerPoint until this got fixed.