Portuguese established companies are required to issue invoices using a certified invoicing software when their annual revenue has exceeded €100.000 in the previous year. This threshold has changed to €75.000 in 2019 and will decrease to €50.000 in 2020. At this moment it cannot be 100% confirmed that the Portuguese invoicing software rules and procedures apply to non-resident companies that have a direct VAT registration in Portugal but this does seem from the published text.
Source: pincvision.com
