Pam Peterson from the support team has elaborated on the PDK user licensing in Dynamics GP 2013 in her article below.
This product (PDK) comes when the customers purchase the business portal add-on for this product called Project Time & Expense. So when a user purchases the Starter Pack (which comes with the Project Time & Expense), the customers get unlimited user licenses for the PDK module.
When the user enters the registration key into the PDK application, the customers will have 9999 PDK licenses to use. However, note that in order to use this module effectively, the customers would need the Project Accounting module which comes along with the Extended Pack.
I hope this information is useful for partners and customers.
Until next post!