Print out the document.
Give it to them to read and stand next to them the whole time so they don't copy it.
Take the document back off them.
Hope they don't tell anybody what it said.
In all seriousness, if these documents have value (i.e. a person will get some advantage by distributing or selling it) then, sooner or later, somebody will do this no matter what you do. If the document doesn't have any value then none of this matters.