Acrobat X Standard keeps forgetting digital ID when the application is restarted

I have Acrobat X Standard and I'm attempting to add a digital ID with a PFX file. I add the ID in the digital ID window (Digital ID Files --> Attach File) and it shows up and is usable for as long as Acrobat is open, but once I close and reopen Acrobat, the ID is gone. How can I prevent it from disappearing?