I opened this issue up on github a couple weeks ago:
It seems like PrinceXML has much more accessibility capabilities than we are currently using. Selecting a PDF that style that includes more accessibility options might be good in the long run.
Even if we continue to use the current PDF for Print exports, there may be a strong case to transition to PDF-UA for PDFs for digital distribution. PDF-UA was a profile of PDFs created specifically for accessibility.
I have been able to download Pressbooks and run them through prince using the PDF-UA command, and although I get errors (usually specifically around correct nesting), it creates much better documents that require the least amount of remediation. Ned taught me how to run prince from the command line in this thread, and thats how I can confirm that this will probably work.