If you have a group of users who need access to these documents and you want no other users to access them, my suggestion is to place them in their own library or site (or even site collection if there's a lot of content). This way is easier to manage security
as you can manage it in one place: the library or site, instead of on each file.
This way too, if no other user has read access to the library or site, they won't see the list or site unless you have explicitly linked to it. If users try to access the content they will get an access denied error. Search results are security-trimmed,
which means if a user does not have permissions to read or edit a file, the file will not appear in their search results.
