Born in August 2012 the global ISO standard 14289-1:2012 - commonly known as PDF/UA (Universal Accessibility) - defines the requirements that an accessible PDF document has to conform with.
Additionally PDF/UA also determines software requirements - no matter if it is an authoring tool, a PDF viewer, an assistive technology or an evaluation tool.
The PDF/UA Competence Center so aptly sums it up:
"But what is PDF/UA, really? It’s not software; it’s simply a document that describes the correct use of another document (ISO 32000) to achieve a certain outcome (ensure accessible PDF files)."
- Worth reading: "PDF/UA in a nutshell - accessible documents with PDF"