The mail and attached documents copies become saved in ‘Documents’ tab of the Order after each mailing.
There is 'Attach to Invoice' option in the tab grid to attach the documents added manually for the mailing together with invoice.
The default state of the checkbox is controlled by OrderAttachDocumentToInvoice setting. Read more...
Note
The documents are not attached when issue a Credit Note (inverted invoice).
Mail the documents directly from the 'Documents' tab.
Use 'E-mail' column for selection and 'E-mail document(s)' button for mailing. The choiced documents are attached to the mail-form where user can type mail body, change address and e-mail.