As much I like Coinbase I would never tell anyone to trust online wallets with their BTC. First you need to download a wallet on your computer, its not hard really. Multibit, or Armory would be good. Then transfer all your BTC to your offline wallet. Now you can make your PDF backup. 
A word about Multibit. It does not, I believe, come with an option to encrypt itself so you would probably want to store it in an encrypted container like a file generated by True Crypt. I hear there are other encrypted container making software out there too.
A word about Multibit. It does not, I believe, come with an option to encrypt itself so you would probably want to store it in an encrypted container like a file generated by True Crypt. I hear there are other encrypted container making software out there too.