If you are trying to protect it from the user that's admin on the box, then it's quite a difficult proposition because they can undo whatever your product does.
- What about saving the file to a different extension, e.g. via word/excel?
- What about I write some code and call it winword.exe, would you let it open? If so, my code could save, transmit etc. the contents.
- How about if I were to write an excel macro to save the contents elsewhere?
- If I have physical access to the machine that has the file (e.g. on hard disk, on USB), I can always attach the medium to another machine and copy, or better yet, just take the hard disk on my way home, copy everything, and bring it back the next day :-)
You might also want to read 10 immutable laws of security, some of which may be applicable in your case.
Verifying the file signature helps when someone attempts to 'save as' or 'copy' in the original format, but not so sure when they change the format (e.g. doc to txt or even an older doc format). If the signature also depends on the content of the file (as opposed to just the format), then the question is whether it can track someone making changes such as making some changes to the content (such as adding spaces at the end, or even bogus content) so that the file signature would change but the essence of the document won't. In the case of office documents, one can typically change the physical file without changing the document contents.
I decided to do something like this because of malware related issues and IP theft concerns at my father's company, and we ended up removing all removable media from the computers, put locks on the physical computer boxes, disconnected the network from the Internet and then made sure that there were usually enough eyes, because he had trade secrets, as well as customer data, that we couldn't lose. (Although this was done in the last century, you can still turn around a workstation or a desktop and see that there are two loops for someone to put in a physical lock and take the key!)
Of course taking a photo with a digital camera always beats all such programs and protections, but very much like in our case, I'd assume that that's outside the scope and perhaps acceptable 'risk'. If not, put on cameras where the machines are (which is actually what my dad did later although he didn't necessarily do it for all machines!)
Hope this helps.