The Constitutional text is very broad:

The President … shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.

So it looks rather absolute, for Fedral crimes. However, the real situation is complicated. This is just one clause in the Constitution, while the President is supposed to be bound by all of it. So, presumably, he can’t exercise his pardon power in a way that violates something else in the Constitution. If you go deeper into the Federalist papers, it’s quite clear that the Founders held that no man should be his own judge, and a self-pardon effectively does just that.

Here is a good write-up, although I do note it was written before the Supreme Court put their thumb on the scale and said he could do whatever the hell he wanted, as long as he doesn’t get impeached for it:

protectdemocracy.org/…/the-presidential-pardon-po…