A while go a question was posted on a national subreddit about a former politician with great national importance and yet the person asking couldn't find much information on their lifes.
One of the comments was linking to the website WikiLeaks with old documents the US government kept on this historical individual.
This prompted the question: Are leaked classified documents a good source to use and if so are they used while still being classified or is that seen as either unethical or perhaps dangerous to a historian's career and/or reputation due to the very nature of the documents?
In general, every historian should take into account not just what's in a document but the conditions of its production (intended to be seen, by whom, circumstances archived, how otherwise found if not in a formal repository, and so forth). For me, leaked docs like those on Wikileaks are just another condition of production to take into account.
The Pentagon Papers were originally leaked documents. Nowadays, historians use them both for their content -- irrespective of the fact they were leaked -- but also as an artifact unto itself (leaked docs that revealed the extent of federal government deception about the War in Vietnam.)
Another interesting set of documents are previously secret files that have been revealed during tort litigation. Check these out: https://www.industrydocuments.ucsf.edu/tobacco/ (tobacco) and https://www.toxicdocs.org (various toxic substances)