Using Topcon Receiver Utility (TRU), connect in Receiver Managing Mode.
Then press the receiver Information icon as shown below:
That action generates the Receiver Info report. The PAC ID is shown in the 4th line of the GNSS Board section.
Note: If a receiver board is replaced, the serial number database must be updated so that the serial number and PAC are related. The PAC is essentially the serial number of the GNSS board.