there's little you can do to "add security" to a document within office.
there is a feature to add an edit read/write type block to a file, but i think that only covers people making changes.
easier option... would be to add protection to your computer.
it's much easier to lock up a user profile... as opposed to a file, once someone's already on your comp.
also there is a myrad of encryption software available. just search for file encryption... on google. probably some that are free.
that'll basically make it so... the file location can not be seen, or accessed. ...without a password or some such.
could also encode or encrypt the data/text within the document.
there's websites that encode text. ..ciphers, or pages where you cut and paste what you typed... into a form, and it spits back out a miss match of gibberish, that's based on the encrypt key of that page... or type of whatever... then normally you have to go back to the site... to un-code it... cut and paste the gibberish in the box. and it spits back out the actual ...readable text.
google... cipher or websites for encrypting documents. or something like that
hope that helps