On the Home screen, tap Invoices, and then on the Invoices screen, tap the invoice that you want to email. 

Tap Send Email, and then on the Email screen, tap Send.

To delete an email address, tap the Delete icon next to it, and then tap Delete.

To add an email address from the address book, tap Address Book and then tap it on the Device Contacts screen.

To add an email address, tap Add, type the appropriate email address, and tap OK.