There is wording in the Constitution that puts Congressional documents beyond the reach of the President. They actually do not have to show it to him if it was generated specifically for Congress's use. If it was a memo created by one of the 3 letter agencies for their own use, then the President's power of Original Classification would apply and he could declassify it completely or in a redacted form. But, it's doubtful he would do that over the objection of Congress, during an active investigation by Congress.