While in read-only protected mode, select all text, and copy/paste into a new document.
The text of the document is not the threat but any macros and programming behind the document are threats. So, if you only need the text, copy that.
But, it is often not that simple. Some documents (like Excel) require macros and some programming to make them useful. At that point, you need to either:
- open the document in the most up-to-date document reader available
- open the document in a reader that is not the most common and targetted reader (something other than MS Office, like LibreOffice)
- open and edit the document in on online reader (like Office 365)
- open the document in a sandbox (like a Virtual Machine) that can limit the impact if the document is going to do something malicious
These options do not check that there is no threat, but it severely limits what impact the threat may have.