You give a different version of the document to each person, with unique typos in each one. If the document gets leaked, you know who did it - the typos form a "signature".
If you release multiple copies of the document, each with a slight variation, then keep track of who got which version, you can (potentially) figure out who leaked the document.
I posted elsewhere, but is more appropriate here: The document itself struck me as checking for leaks. (Misspelled "hight"? No stamp?)