Hey! Sorry, I saw your message this morning but I was distracted so I thought "alright, I'll come back to that in a second" and then forgot all about it! I fail...
PC stands for Pokecommunity, and a PC Supporter is someone who has donated money to the forums. All the information is here.
See what I did just there? Put the link in the word "here"? I know it's nothing, but I'm very impressed with myself because I only just learned how to do it lol