Impact on Users
PDFs cannot be made fully accessible, as it does not yet have all the features of HTML. Therefore an equivalent must always be provided There are specific features that must be implemented within a PDF to optimise access for people with a disability, especially for people who are blind or have low vision.
PDF Accessibility Features
- Tagging content elements within a PDF creates a logical document structure for assistive technologies to interpret. Having a logical structure means users can understand the reading order of the document and are provided with extra information about content, such as text, images and tables. Non-tagged PDFs are unreliable and assistive technology may be presented with incorrect information, an incorrect reading order or content may not be read at all.
- Logical reading order
- A logical reading order or structure is required for screen readers and other assistive technology to move through the document in an order that is consistent with the meaning of the content.
- Text descriptions for all meaningful images (ALT text)
- Images (including graphs, diagrams, scanned text) cannot be interpreted by a screen reader. Alternative text communicates the image content or purpose to users of screen readers or users with a cognitive disability. Images of scanned text are inaccessible because assistive technologies cannot read or extract the words and therefore users cannot access it. Additionally, text is unable to reflow, which means the text is unable to wrap, or adjust on zoom. Scanned images are also not searchable when a user is looking for information in the document.
- Tables that use the appropriate markup tags
- Tabular information (data in a table) must be presented in a way that preserves the relationship of content in different cells, even when users cannot see the table or the presentation format. Using table markup tags, the logical relationship between text, numbers, images, or other data that are represented in columns and rows of the table, is conveyed to the screen reader user.
- Bookmarks and heading tags
- Bookmarks and heading tags allow users to locate content more easily by providing a hierarchical outline of the document content, rather than reading through many pages. This outline is especially useful to a person with a cognitive disability or a screen reader user. Bookmarks benefit all people by providing users with a mechanism to go directly to the page or section they want.
- Documents with additional navigational aids such as a table of contents provide an easy way for users to move around a document and scan pages for information. Page numbering provides a mechanism for users to orient themselves and find information and is consistent across different PDF readers.
- Document language
- By identifying the language of the document, assistive technologies and conventional user agents can render text more accurately. Screen readers can load the correct pronunciation rules and visual browsers can display characters and scripts correctly. As a result, users with disabilities are better able to understand the content.
- Providing name, role, state, and value information for all form components allows assistive technologies to gather information about and interact with form controls in a PDF. Form components may include text input fields, checkboxes, radio buttons, combo boxes, list boxes, and buttons. Without this information form fields may not be controllable by assistive technology users. In addition form fields with tooltips can be read by screen readers to provide extra information.
- Security features
- Most PDF security settings are not compatible with assistive technologies and will prevent access to the document content. Text access for screen reader devices should always be enabled and/or security settings be set to ‘no security’.
- Accessible alternative
- Support for PDF accessibility features over various assistive technologies is varying and incomplete. Some features convey semantic meaning that is not supported by all assistive technologies (such as screen readers). For example, a tagged heading allows a user to navigate the sections of a document quickly, and a tagged paragraph identifies paragraph blocks. These features are not yet consistently supported and therefore an equivalent accessible alternative (HTML, Word, Text or RTF) must also be published, with a link to the alternative next to the PDF link.
- Download link
- Users should be able to understand the purpose of each link so they can decide whether they want to follow the link.
- A list of links on a page can be provided by some assistive technology, and descriptive link text will help users select a link from this list.
- Keyboard users are able to tab from link to link and understand the purpose of the link.
- Descriptive link text helps inform users of the content to expect and the format it may be presented in (e.g. a downloadable document such as a PDF or Word document), enabling them to make an informed choice prior to selecting the link.