MS Word: Document-level Security

I am looking for a method to restrict access to a Microsoft Word document. The goal is to allow access to specified users via specified PCs.

Password-protection will not help me because the password can be shared with an outsider. I have researched Microsoft’s Rights Management tools and I think this is essentially the same situation as password-protection.

The real challenge is that the MS Word document needs to be edited by the user and emailed to another (authorized) user, so it must be possible to transfer the MS Word file from the PC while retaining the security settings.

Thanks in advance for your suggestions.