It doesn't work that way. The PIC and the PC communicate over a protocol you choose or even make up. You implement the protocol in the language of choice at either end. Your PC program sends a message to the PIC program and vice versa. You do not execute any instructions across the link. So on the PIC you write a program that does something like "when I see the letter D has been sent to me, I send back Hello World". On the PC end the program does something like "send a D then get back a message and then display it".