It is possible. It is far from simple. Note also that hidden information in a document is always there with the potential to be seen by someone you do not intend to see it.
I would suggest that anything going to your clients be in the form of pdf rather than Word.


Your profile is showing you on Windows XP with Office 2016. This should not be possible.


I guess I would have a paragraph style set for the information to be hidden from clients. You can easily turn the "hidden" font formatting for this on or off. You could record a macro to do so.