These three resources cover expert-level PDF accessibility.
This 2021 course from LinkedIn covers how to create and test accessibility in PDFs. The course costs $30.
https://www.linkedin.com/learning/creating-accessible-pdfs-14445392?trk=lynda_redirect_learning
This 2023 course from LinkedIn goes into more detail about advanced topics in accessible PDF creation and editing. The course costs $30.
https://www.linkedin.com/learning/advanced-accessible-pdfs-21974253?trk=lynda_redirect_learning
This 2022 checklist from the Orbis Cascade Alliance walks users through the process of creating PDFs and other digital course materials. There are three levels of accessibility practices described: bronze, silver, and gold.
https://www.orbiscascade.org/programs/dux/asg/pdf-and-digital-materials-checklist/