PDF/UA (Universal Accessibility) was published in July 2012 as the ISO 14298-1 standard and designates a variety of the PDF standard for creating accessible PDF documents.

PDF/A-3 as the preferred ZUGFeRD format​

The Central User Guidelines of the Electronic Billing Forum of Germany - best known by its acronym ZUGFeRD, which resembles and is pronounced like the German word for a draft horse - describe a new document and file format for the exchange of electronic invoices. The initiators' goal is to establish a new standard for billing that will be published as a national norm in Germany and then submitted to the European Committee for Standardization (CEN). PDF is playing a key role in this program, especially since the visual depiction of the billing data is generated using a PDF/A-3 document, with an invoice data file in XML format embedded within it at the same time.

A good read: PDF Association clears the air​

For many businesses, the PDF/A format provides the ideal basis for long-term archiving that is as uncomplicated as it is secure. Originally designated PDF/A-1, this standardized version of the conventional PDF format was established as the international standard ISO 19005-1 in late 2005 and has been available as an ISO standard in its extended versions PDF/A-2 and PDF/A-3 since July 2011 and October 2012 respectively. The big advantage of this format is that it guarantees reliable reproduction over time regardless of the application or system that was used to create the file.