Are you not paying attention. First, the charges against the president have nothing to do with rather the documents were classified or not. Second, there is no record of that declassification order, no notification of the departments where the records resided, no memo, not even a witness from his staff.
The biggest charge here is obstruction of justice. Trump failed to produce the documents and it appears, he actually moved them to avoid turning them over. He has no attorney client privilege because he don't own the documents in question. Hell, if you rob a bank and turn the money over to your lawyer, does that mean the money is protected due to attorney-client privilege? How is this different? Nor does he have executive privilege for the same reason, he doesn't own the damn documents.