Being a lawyer, I can't really give you actual legal advice on how to do this stuff without you being my client, but i can explain a very very generalized process.
Note that if you are not a lawyer, you will end up spending some time reading rulebooks and filling out forms that lawyers know how to fill out. There are usually legal aid folks/etc in most state courts that can help.
So let me give you an example process (Again, this is not legal advice, just an example process):
1. Open a small claims case against the pseudonym of the paypal person (to be filled in later), or what they claim their name to be if you have it, in the state you believe them to be in (assuming that state allows subpoenas in small claims. If not, you may have to open a real case in their district court, which is more expensive and requires more paperwork). This will require service of process, which will be initially hard, but there are fallbacks in case the person is deliberately making themselves unreachable, like publishing service in newspapers, etc.
2a. Start by getting a subpoena from the small claims/etc court, see if that's enough to get what you want.
They may want a subpoena from their local court. In that case
2b. Contact the relevant court for paypal (northern district of california, I believe), and get them to issue an out-of-district subpoena. For the central district, you can see the exact process here: http://www.cacd.uscourts.gov/court-procedures/filing-procedu...
It should be roughly the same process for northern district (the form numbers are the same, though the form content will say northern district instead).
Note that the envelope should be addressed to you.
They will assign a miscellaneous action number, and mail you an endorsed subpoena.
3. Send a copy of the endorsed subpoena (keep the original since you are likely to end up playing mail tag a few times) to the address paypal provides.
Again, i don't have this address handy, but as a registered corporation, you should be able to lookup where they receive service of process. They are required to have a registered agent that accepts service of process.
If they fail to respond, file a motion to enforce the subpoena (this is covered in the URL linked above in the case of northern district, if it's another local court subpoena, they will have a similar way).
They do have a fraud investigation team which you could contact with the subpoena first, and see if they will respond.
They may work only with law enforcement.
Again none of this is legal advice, there may be intermediate or more steps, i'm not responsible if the steps are not right or in the right order, or if following these steps causes you to lose all your legal rights, etc.