NEW - Biden says "The president wrote a very generous letter," referring to Trump. "Because it was private, I won't talk about it until I talk to him. But it was generous."
This taken from the US Military Voice Telegram Channel.
I'm betting the 'generosity' was the deal Biden cut with Trump...
You pardon people for crimes they have committed, whether there's been a conviction or not, whether there's been an indictment or not, whether they've been charged or not. The individual can choose to accept the pardon or not. Accepting it is seen as an admission of guilt.
A notable example would be Jimmy Carter preemptively pardoning the 500,000 Vietnam draft offenders (causing many of them who had fled abroad to return home).
More on-the-nose, perhaps, would be Ford's pardoning of Nixon. His very broad preemptive pardon literally reads "a full and unconditional pardon for any crimes that he might have committed against the United States as president."