PDF/A-1a is with conformance level A (for "accessibility"), and must adhere to all of the requirements of the PDF Reference as modified by the ISO 19005 specification. It requires structural and semantic properties to be preserved. Level 1a uses “Tagged PDF” and Unicode character maps to preserve the document's logical structure and content text stream in natural reading order. The following features are used: - Language specification - Hierarchical document structure - Tagged text spans and descriptive text for images and symbols - Character mappings to Unicode The purpose is to improve accessibility and make the content accessible for screen readers.
The currently supported compliance levels can be found in the How to Comply with PDF/A Standard article.