HI. if I purchase a launch permit online, can I immediately download it as a pdf, or will I have to wait to receive it in the mail?

Yes you get a permit you can print out on paper, (I put mine in a zip lock bag) and they will send you the official by mail.

If you pay you play.

I bought mine online and had it to show the ranger when he check. Thanked me and said i was good to go

Better to wait and receive in mail and then print it.