Similar to (x)HTML and CSS, which are the technologies use for traditional web pages, PDF documents may be designed with accessibility in mind. Today, most of the public PDF documents are inaccessible.
A typical example of an inaccessible PDF is a scanned document originally consisting of mostly text. Assistive technologies, such as screen readers used by for example visually impaired users, are designed to read normal text. However, if a PDF is scanned, even when it consist of only text, it is perceived as an image. These images can not easily be interpreted as text by a screen reader, and the document would for the users who require text to be read out loud, be completely inaccessible.
PDF documents may contain structured elements, often referred to as “tags”.
Tags in PDFs are used to denote the semantics of the text such as headings, images, links etc. As an example, when headings are properly tagged, users can navigate through the document using the chapter and section headings. In contrast, if the PDF is untagged, headings and paragraph texts are indistinguishable and navigating using the headings is not possible. Clearly, for large documents, navigating through the document is challenging if not close to impossible.
It should be noted that as with (x)HTML, tags can be used improperly. Letting a PDF be tagged does not by itself guarantee
that the document is barrier free. This is rather prerequisite for accessible PDF documents.
Access to content.
Occasionally, PDF documents restrict users to copy the text. Unfortunately, by enabling this option, it affects the assistive technologies in a negative manner. These technologies, such as screen readers, are restricted from extracting the content.
When a text is read out it is essential that it is read in correct language / dialect as the text is written. As an example, synthetic reading of an English text with German pronunciation will be impossible to understand. It is therefor important that the language is specified in the PDF so that the proper pronunciation could be applied by screen readers. Most PDFs are missing language specification.