Congress does not classify or declassify documents. It authorizes the Executive Branch to do these things. By definition, the President of the United States (the only Executive Branch official named in the U.S. Constitution who has defined Executive Branch powers) has plenary authority over the entire Executive Branch and has the sole discretion to accept or override any such decision related to classifying or declassifying documents.
I would add that the judicial branch has the Constitutional authority to obtain classified information and share it with jurors if needed for a trial, regardless of any statute.
The charges against Trump are simply absurd, which is why they have never been brought or even threatened against any other former President. And any President can simply pack up documents and say, "these are mine" while still in office.