Let's review accessible pdf characteristics, practical measures of accessibility with a review of common tools, and what is meant by, "technically accessible".
Accessible PDF Characteristics
From, “Adobe, Accessing PDF Documents with Assistive Technology, A Screen Reader User’s Guide,” pages 2 – 3:
"All accessible PDF documents have the following five characteristics:
A logical structure and reading order… Alternate text descriptions for figures, form fields, and links… Navigational aids… Security that doesn’t interfere with assistive technology… Fonts that allow characters to be extracted to text..."
Practical PDF Accessibility
Use methods provided by the PDF Association (PDF/UA) via their Matterhorn Protocol which provides techniques to implement the ISO PDF/UA standard.
Also implement the expectations of the client whether commercial or government agency as well as guidelines provided by W3C WCAG.
The ISO PDF/UA Standard is the official international interpretation of technical accessibility for PDF files and is harmonized with the U.S. Rehabilitation Act of 1973.
If you follow the guidelines from W3C and use techniques recommended by the PDF/UA and government agencies such as HHS and VA, the resulting electronic output will be accessible to people who rely on assistive technology (AT).
Free Learning Resources (to list just a few)
Adobe, Creating Accessible pdfs, https://helpx.adobe.com/acrobat/using/creating-accessible-pdfs.html
Tagged PDF Resources, http://taggedpdf.com/508-pdf-help-center
U.S. Department of Veterans Affairs eLearning Support, http://www.section508.va.gov
U.S. Department of Health & Human Services eLearning Support, http://www.hhs.gov/web/508/accessiblefiles/pdf-required.html
First, the Rehabilitation Act of 1973 describes the desired outcome scope and enforcement provisions.
Second, the Matterhorn Protocol ensures a technically accessible document by meeting the provisions of the ISO PDF/UA standard.
Third, use techniques as described by Adobe, HHS, and VA.
Fourth, use industry best practice to select tools to perform remediation and validation: Adobe Acrobat Professional (remediation) and PAC v2 or the Validator for validation.
Adobe Acrobat Professional provides the means to work with the Content, Reading Order, and Tag trees as well as the ability to modify content, text size, type, and color, and add hyperlinks, alternative text (images and hyperlinks), and table summaries.
PAC v2 tool is an analysis engine with a reporting feature offered for free by Access for All, a non-profit organization. PAC v2 provides validation automation by checking for all of the Matterhorn Protocol conditions.
The Validator tool is a free evaluation tool for authoritative validation of pdf files from NetCentric. The Validator is the analysis and reporting engine of their CommonLook Global PDF checker.
pdfGoHTML plugin is provided for free from Callas software. The Adobe plugin allows a one-button conversion to an HTML view of the pdf document. Note: the PAC v2 tool has a similar utility offered through the Screen Reader Preview option.
Does Not Mean Completely Accessible
Knowledgeable human review of tags is still required because no single tool automates remediation for all issues.
Reference the video tutorials found on the 508 PDF Help Center – Tagged PDF website.
Using accessible designs in the source application, such as Word, InDesign, Excel, or PowerPoint, is the key activity to obtain usable and accessible pdf files.