Another unanswered question. This one from 10 years ago.
There are currently two tags for converting HTML to PDF. CFdocument and CFHTMLtoPDF.
CFdocument will not generate a tagged PDF. Adobe doesn't plan on fixing that.
CFHTMLtoPDF was introduced in ColdFusion 11. It generates a tagged PDF --- except the image descriptions (ALT attribute values) are not being exported to the PDF. As of this reply the Adobe intends to fix that in the next release.
I suggest using CFHTMLtoPDF but avoid using images. If images must be included, you might caption the images with plain text (in the manner of newspapers or news magazine) and use the aria-describedby and id attrbutes to anchor the captions to the images.
Note that you must start with an accessible HTML document with strict (semantic) HTML and perhaps some ARIA.
I hope this suggestion helps someone.