PDF/UA, also known as ISO 14289-1, is an international (and highly technical!) standard that describe how a PDF document should be structured in order for its content to be accessible to assistive technology users. Other standards (actually ISO working drafts) determine how assistive technologies, PDF renderers, document authoring tools and conversion processes should leverage this standard.
Unfortunately, you’ll need to buy the document from ISO in order to read it; please keep in mind that this is a technical document that requires in dept understanding of how the PDF document format works, and it is not geared towards end user willing to make their PDF documents accessible. for the sake of completeness, some institutions (e.g. universities, research centers,) might provide you access to such documentation for free: which documents are offered under what conditions and how to access them is determined by each subject individually.