I found this document:
Burdick v. United States, 236 U.S. 79 (1915)
It seems that for a pardon to be considered, the recipient must accept it.
If the recipient does accept it before conviction or confession, it is considered an admission of guilt.

My question: if a person proclaims innocence and does not accept the pardon thus forcing a trial, can he later accept the pardon if he is found guilty?

Also: Can a pardoned person be compelled to give testimony or face contempt charges?