My understanding is that the pardon removes his ability to invoke the 5th, since he cannot be prosecuted for the acts covered by the pardon, which puts him in jeopardy for perjury or contempt, and gives Congress the chance to seek the truth for the value of showing the world the truth.
Watch Fauxi claim the pardon was invalid since it was signed via autopen, so he can still claim the 5th.
Then when charged, will claim he was pardoned.
Get popcorn!