There's a good primer on classification and how that interacts with the law (various statutes, most notably the Espionage Act of 1917 - which says nothing about whether a document is 'classified' or not) over on Lawfare. Written when the question was around Bolton publishing a book that might (or might not) contain classified info - but it might be useful here.

SECRECY & LEAKS: The Law of Classified Information: A Primer