Does 64-bit compatibility matter with files/documents? macOS Catalina runs only on 64-bit, so when I reload my existing Microsoft Office files, will they be "read only"?
The 32-bit/64-bit description refers to executable code, rather than data. Applications and other executable code will contain instructions that are either 32 or 64 bits wide. 

However, file formats are largely unaffected by this. Almost all applications will open the same file data regardless of the application's bit-depth. So, MS Office documents can be opened by both 32 and 64-bit versions.

It's possible that new software may introduce new data structures than are unreadable by older software, but that's a different issue.