The single most intriguing issue with the concept of PDAs is the possibility that every user can have a Trusted Computing Base (TCB) under his control which does not include resources controlled by others. That is, if the system is designed in such way that the user's PDA constitutes his TCB, then keys and credentials can safely be stored in it. More specifically, a trusted PDA can act on its owners' behalf when the owner delegates access rights. In particular, when it comes to the case of binding a public key to a human, the TCB consists only of the user's PDA. This, of course, stems from the use of public-key technology rather than offline delegation per se.
Recall the scenario with Alice delegating authority to Bob while speaking on the phone. In this scenario, FR is not part of the TCB because it is ``only'' used to store files. The result is that FR is unable to impersonate the user, neither when interacting with other instances of FR, nor when interacting with other users.