Edicom mentioned in this post that in April 2014, electronic invoicing finally became mandatory for all companies registered in Portugal and that as as from e-invoicing is mandatory for all companies as from 1 January 2015.
Some further research learned that the Portuguese Tax Department forces companies in Portugal to generate an electronic standard document (saft) for einvoices. This document, must also be signed with an electronic signature. And finally, the e-invoice has to be sent to Tax Administration looking for their validation. It is a model that closely resembles the Latam system which is focused on tax control.
And even though, at the end, companies are allowed to send the invoice to their partners by electronic means or on paper, the only mandatory requirement is the use of a certificated software for generating the einvoice and the declaration of the invoice towards the Portuguese Tax administration.