Structure tree

The structure tree is where a tagged PDF keeps its tags: a single hierarchy, from the document down through sections, headings, paragraphs, lists, tables, and figures, describing what every piece of content is and what it belongs to.

It is the part of the file assistive technology actually reads. The visible page is a set of drawing instructions with no meaning attached, so a screen reader walks the structure tree instead: the tree is what makes "heading level 2", "row 3 of 12", and "figure with this description" possible, and it also fixes the reading order, which need not match the order things happen to be drawn on the page.

A PDF with no structure tree is untagged, whatever else is right about it. Content the tree does not cover is invisible unless it is marked as an artifact instead.

Learn more

• Tagged PDF Best Practice Guide: Syntax - the PDF Association's developer guide to the structure tree
• Tagged PDF
• Artifact
• Reading order