From 1 February 2026, KSeF will allow invoices with attachments, but only in a structured format as part of the XML file (FA(3)). Attaching files such as PDF, JPG, or scans will not be possible – those must be exchanged outside the system. Attachments are optional and require prior notification in the e-US portal. The function is available to all taxpayers, but is particularly useful in industries with large data volumes (e.g. utilities, fuels, telecommunications).