PDF version of Invoice attached to email

Customer would like to receive a pdf copy of their monthly invoice by email. Currently, the invoice information is contained in the body of the email and they have to convert it manually to a pdf for their records. They can get the pdf by going to the Purchase History page and clicking the inline button to the right of each charge, but that requires many more clicks than opening an email they already get. If the pdf version were attached to the current invoice emails, it'd be perfect. Thanks!