ShadowPrint is not able to print to virtual printers such as PDF printers or Microsoft XPS Document Writer. This is because these printers need some form of user intervention to confirm print jobs. ShadowPrint was designed to need no user input once it has been configured.
If a virtual printer is selected, the ShadowPrint service will send print jobs to the virtual printer. They will queue until the services limit is exceeded which will stop the ShadowPrint service.