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".

Sometimes a typo is just a typo though.

Everybody involved gets a slightly different unique version of a document. It would be clumsy to do it with typos, better to change the justification.

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.