Or you can get Evernote.
It comes with an email address that you can forward the email too. It won't be saved as a PDF though. On the other hand, if you load Evernote on your computer too it will automatically sync there, and Evernote is a lot more searchable than a folder full of PDFs.
The other thing I do is use Printopia (Fingerprint for PC users). This allows you to print to any printer (or folder) on your computer. It uses the built in Print option in most apps. You have to have your computer on and the program (Printopia or Fingerprint) running on the computer.
I set up a Receipt folder in Printopia on my Mac. When I receive an email receipt on the iPad, I just print it to that folder. The result is a PDF of that email where I want it.
Adobe