Seal the document in a glass case before giving it to the client. As a
precaution, it's generally best to include a chemical in the sealed case
which, when mixed with air, causes the enclosed paper to be destroyed
immediately if the glass case is broken.
Barring such extremes... different degrees of security -- but nothing is
foolproof -- can be attained depending on what version of Word you're using.
What version of Word are you using? Look at Tools - Option - Security and
Tools - Protect Document to see some of the Word-based options available.