A tagged PDF includes hidden accessibility markups that, when properly applied, help to optimize the reading experience of those who use screen readers and other assistive technology (AT). Meticulous tagging is a crucial component of achieving a truly accessible PDF. A properly tagged PDF can also re-flow to adapt its presentation to different screen sizes, for example to provide a high-quality experience to users of smart mobile devices.
The illustration below shows a small portion of an accessible PDF document, with the tags along the left margin.
Tags provide a logical structure that governs how the document content is presented through assistive technology. Each tag identifies the associated content element, for example paragraph <P>, heading level three <H3>, list item <LI>, image <Figure>, or table data cell <TD>. The order of the tags defines the reading order. Purely visual elements, such as images that provide visual interest but no meaning, are tagged as background artifacts and disregarded by AT. Tags for meaningful images such as graphs must include alternate text that clearly conveys the same meaning as the visual representation.
It is possible to automatically add tags to a PDF, but the result is never satisfactory without careful, knowledgeable human inspection, testing and corrective actions. A tagged PDF has very little room for error. Even small errors in the logical structure can render the document incomprehensible to assistive technology users.
While properly constructed tags are essential for an accessible PDF, on their own tags are not sufficient. Careful tagging must be used in combination with other aspects of document accessibility such as properly defined document language, meta data, font size, font embedding, color contrast, and bookmarks to achieve a truly accessible PDF.