It's important to remember that a presidential pardon reduces the ability of the recipient to invoke his Fifth Amendment privilege against self-incrimination.

If you have been pardoned for federal offenses, you can no longer invoke the possibility you might be prosecuted for those offenses as a basis for refusing to testify, since the pardon bars that possibility.

And the pardon of course cannot bar prosecution for perjury in testimony compelled after the issuance of the pardon.

It would still be possible to assert the Fifth as to the possibility of incrimination under state law, since a federal pardon does not cover state offenses.