Actually your walking down the wrong path for Accessible PDF.
WCAG's the cat's meow for your HTML work.
Where the rubber-hits-the-road WCAG does not cut it for PDF.
For PDF you work to ISO 14289-1, PDF/UA-1.
A PDF's Structure Tree is what AT uses to "work" the PDF for AT users.
The Structure Tree "Shall" have a document root (required by the ISO standards for PDF and PDF/UA).
Good to do -- Reset your workflow.
Study and make use of the resources identified below.
For those who desire to / need to provide accessible PDF success is predicated in having and understanding the essential reference material.
ISO 32000-1 (14.8 in particular).
Document Management – Portable Document Format – Part 1: PDF 1.7, First Edition
Adobe (Jul 2008)
This document is an ISO approved copy of the ISO 32000-1 Standards document. By agreement with ISO, Adobe Systems Incorporated is allowed to offer this version of the ISO standard as a free PDF file on their own Web site. It is not an official ISO document but the technical content is identical; the page and section numbers are also preserved.
ISO 14289-1 (PDF/UA-1)
This you will have to purchase this from the ANSI Web Store.
@ The PDF Association
The Matterhorn Protocol.
The resources for Accessible PDF at the AIIM site.
PDF/UA-1 Technical Implementation Guide: Understanding ISO 14289-1 (PDF/UA-1)
PDF/UA-1 Technical Implementation Guide: Understanding ISO 32000-1 (PDF 1.7)
Achieving WCAG 2.0 with PDF/UA
(WCAG 2.0 for PDF IS PDF/UA-1)
And the read-only older Accessibility Forum of the AUC site.