Presidential or Legislative Pardon of the President
Under the fundamental rule that no one may be a judge in his own case, the President cannot pardon himself.
If under the Twenty-Fifth Amendment the President declared that he was temporarily unable to perform the duties of the office, the Vice President would become Acting President and as such could pardon the President. Thereafter the President could either resign or resume the duties of his office.
Although as a general matter Congress cannot enact amnesty or pardoning legislation, because to do so would interfere with the pardoning power vested expressly in the President by the Constitution, it could be argued that a congressional pardon granted to the President would not interfere with the President’s pardoning power because that power does not extend to the President himself.