You can only block out users from file access by making it a secrert where those files are. As your app runs with the Windows account of the user, he needs access and modification permissions anyway, unless your application impersonates another users.
There are such Windows API cals, that would alow that, but it has it's flaws and riscs, once your application runs with another account users don' eetheir dcuemnts folder in a GetFile() dialog, for example, also they can get at the docx from there, if they just find out.
In the end your file security can only be better or differing from the OS, if you do encryption of files only your application decrypts. But even with sich options first think about how to solve your permission design in normal ways of folder/file permissions. If you need fina granular permission control you better have a system where people need to check out and in files, eg Sharepoint or even some source control management. And documents themselves can't contain parts a user only should be able to read and parts a user can edit. You never mentioned such a demand, but if you had it, it would be reason to even rethink document composition.
Bye, Olaf.