The PDF/UA standard defines technical requirements for universally-accessible PDF documents by identifying a set of relevant PDF functions (including text content, images, form fields, comments, bookmarks and metadata) based on ISO 32000-1 (PDF 1.7) and specifies how they should be used in PDF/UA-compliant documents. It does not address elements which have no direct impact on accessibility, such as the compression algorithms used for image data.
Successful access to content within PDFs depends not just on compliant documents, but also on compliant PDF programs and assistive technology. PDF/UA therefore also specifies requirements for these. A brief selection of the main requirements is shown below:
File format requirements
- Content is categorised in one of two ways: meaningful content, and artefacts such as decorative page elements. All meaningful content must be tagged and integrated into the structure tree of all tags within a document. Artefacts, on the other hand, need only be marked as such.
- Meaningful content must be marked with tags and, together with the other tags in the document, create a complete structure tree.
- Meaningful content must be marked with the appropriate semantic tags.
- The structure tree created by the document tags must reflect the document’s logical reading order.
- Only the standard tags defined in PDF 1.7 may be used; if any other tags are used, a role assignment entry must record which standard tag each one represents.
- Information may not be conveyed using visual means alone (e.g. contrast, colour or position on the page).
- A document title must be given, and the document must be set up so that the title (rather than the file name) appears in the window title.
- The language of all content must be noted, and changes of language must be explicitly marked as such.
- Any pictorial elements – whether image objects or other non-text objects such as vector objects or object groups – must have corresponding alternative text.
Adhering to these technical and semantic requirements will create a universally accessible PDF/UA document which a person with disabilities can make use of just as effectively and in just as high quality as any user without disabilities.
Requirements for compliant PDF programs
PDF programs are often the link for people with disabilities between the PDF document to be read and any assistive technology that may be used. In practice, this means that PDF/UA-compliant PDF programs must hand over all content and other information from the PDF document to the assistive software or device, and that the assistive technology itself must make use of all the information it receives, including for navigation, filling out form fields, or reading metadata.
- The software must be able to read and pass on all information stored within the document’s tags and structure tree, or otherwise make all the document’s content accessible.
- It must make all content available to the assistive technology, including structural information.
- It must not limit the functionality of the assistive technology.
- It must allow the assistive technology to recognise the language used and any changes in language within the document.
- It must allow navigation through the document by page number, through the structure tree, or through bookmarks.
- It must not play media content automatically, but rather wait for the user’s command to do so.
Requirements for compliant assistive technology
The term “assistive technology” describes anything which helps or allows people with disabilities or other difficulties to use any kind of hardware and software. People with serious visual impairments may use screen magnifiers. People with restricted mobility can use joysticks or special keyboards for input and navigation. Highlighting the current portion of a document while also reading it aloud using a text-to-speech function can make it easier for users with dyslexia to read a document. Blind users often use a standard keyboard for input and navigation alongside a screen reader or Braille display for output. For all of these assistive technologies to work properly with PDF/UA, they must meet the following requirements:
- They must be able to recognise all structural elements, attributes and key values used in the specification and output them for the user of a PDF document.
- They must allow the user to navigate through the document by page number, through the structure tree, or by using bookmarks.
- They must allow the user to easily set and change the magnification of a PDF document at any time.