Save invoices

Did you get a File icon in the mail as attachment or is the invoice just part of the email?

If its just part of the email, then your only option is to screenshot the email. By Pressing the Home and power/sleep buttons and quickly letting go. The screen will flash and the screen shot will be saved to Camera Roll.

Unfortunately there is no ability to directly save emails in the native Mail App.

Evernote might be a viable option for you. The app and the basic service are free. It accepts emails via an account option you set up, and you simply forward any email you wish to store and it is saved. I use for that purpose on a regular basis (there are also PC and MAC version).

If the Invoice was received via Email on the iPAd and is in PDF format simply touch and hold the file icon in the email and then tap on Open In option to save the file into one the available Apps.

You should be able to add it to either App from wthin the App using their own Add file option. And choosing it form Camera Roll. It will then copy it into the App.