If you read the indictment, most of it is focused on once Trump s out of office, not leaving.
President and ex presidents both have broad authority on what documents they have. After Nixon it was kinda like well you gotta work with us. But the President can tell the national archives to get bent and there isn’t anything they can do about it.
That tacit assumption must be that any documents removed from the archives while he was President have a classification level known only to Trump.