In my experience, it is always a safer practice to convert any finalized word documents into a PDF format prior to filing in the TMF. Although technically, PDF documents can be altered/edited too, the possibility of one accidentally editing a .pdf vs .docx is significantly lower. Converting to a PDF will help avoid the risk of accidental/inadvertent edits when opening/closing the document.
… With that said, of course, I have encountered occasions when a document’s final version is a MS Word file. In those cases, I always sent/kept an e-mail correspondence with the MS Word file attached (with the e-mail content specifically stating that the attached is the finalized/approved version), so that I can trace back when the document was finalized, and in what condition. That e-mail is then filed as a part of the study correspondence… = Audit Trail 🙂
I think the best practice would be to establish an internal procedure (e.g., SOP/Work Instruction) at the organizational level to specify and implement a documentation practice that suits your business needs. There are some good articles and white papers online surrounding Good Documentation Practices that you may be able to use as a reference too.
Hope this helps!