Page 9 of the Explanatory Notes
Additionally, for an invoice to be considered according to the VAT Directive to be an electronic invoice, it needs to be issued and also received in any electronic format. The choice of format is determined by the taxable persons. This would include invoices as structured messages (such as XML) or other types of electronic format (such as an email with a PDF attachment or a fax received in electronic not paper format).
invoices created in paper form, that are scanned, sent and received via e-mail can be considered as electronic invoices.
Source European Commission – explanatory notes
