PDF Tags are a text-only representation of the PDF file which is hidden inside the PDF file. Tags are used to present the content to screen readers instead of the original file. They exist for accessibility purposes only and have no visible effect on the PDF file. In most cases, tags are necessary in order to make a PDF file comply with Section 508 of the US Rehabilitation Act (access for the disabled).

Currently, Confluence supports adding Alt Text to Images that will be read in the UI, however, they're not included in the PDF export. This results in having to duplicate work in possibly large documents for all images to achieve compliance on the PDF.