Logical Structure

Tagged PDFs include a logical structure tree that defines the document’s semantic hierarchy — paragraphs, headings, tables, figures, and lists. Aspose.PDF FOSS for .NET provides the TaggedContent API for building and inspecting this structure, which is essential for PDF/UA accessibility compliance.

Accessing tagged content

using var doc = new Document();
var tagged = doc.TaggedContent;
tagged.SetTitle("Accessible Document");
tagged.SetLanguage("en-US");

Creating structure elements

ITaggedContent provides factory methods for every standard structure element type. Elements are assembled into a tree by calling AppendChild.

var root = tagged.RootElement;

var heading = tagged.CreateHeaderElement(1);
heading.SetText("Chapter 1");
root.AppendChild(heading);

var paragraph = tagged.CreateParagraphElement();
paragraph.SetText("This is the first paragraph.");
root.AppendChild(paragraph);

Structured tables

TableElement creates accessible tables with head, body, and footer sections.

var table = tagged.CreateTableElement();
root.AppendChild(table);

var thead = table.CreateTHead();
var headerRow = thead.CreateTR();
headerRow.CreateTH().SetText("Name");
headerRow.CreateTH().SetText("Value");

var tbody = table.CreateTBody();
var dataRow = tbody.CreateTR();
dataRow.CreateTD().SetText("Width");
dataRow.CreateTD().SetText("612");

Illustrations and figures

IllustrationElement wraps images in the structure tree.

var figure = tagged.CreateFigureElement();
figure.SetImage("chart.png");
figure.SetAlternativeText("Sales chart for Q4");
root.AppendChild(figure);

Structure element types

The API supports the full set of PDF structure element types:

Block-level: Paragraph, Heading (H1-H6), List, ListItem, Table, BlockQuote, Code
Inline: Span, Link, Annotation, Figure, Formula
Grouping: Division, Article, Section, Part

Tips and Best Practices

Always set Title and Language on TaggedContent for PDF/UA compliance.
Build the structure tree top-down: create elements, then append children.
Use SetAlternativeText on figure elements for screen-reader accessibility.
Tag tables with THead, TBody, TFoot for proper table semantics.
Validate PDF/UA compliance with PdfFormatConversionOptions (see Conversion guide).

Common Issues

Issue	Cause	Fix
PDF/UA validation fails	Structure tree missing required elements	Ensure all content has corresponding structure elements
Heading order is wrong	H2 appears before H1	Follow sequential heading hierarchy
Figure has no alt text	`SetAlternativeText` not called	Always provide alt text for illustrations
Table not recognized by screen reader	Missing THead/TBody structure	Use the full table structure API

FAQ

What is a tagged PDF?

A tagged PDF includes a logical structure tree that maps visual content to semantic elements (headings, paragraphs, tables), enabling accessibility tools and content reflow.

Is tagged PDF the same as PDF/UA?

PDF/UA is a standard that requires tagging plus additional accessibility rules. Tagged PDFs are a prerequisite for PDF/UA compliance.

Can I tag an existing PDF?

The TaggedContent API works best with documents created from scratch. For existing PDFs, inspect the structure tree through Document.TaggedContent.

API Reference Summary

Class / Method	Description
`TaggedContent`	Entry point for logical structure operations
`TaggedContent.SetTitle`	Set the document title for accessibility
`TaggedContent.SetLanguage`	Set the document language
`ITaggedContent`	Interface with factory methods for all element types
`StructureElement`	Base class for all structure elements
`StructureElement.AppendChild`	Add a child element to the tree
`StructureElement.SetText`	Set the text content of an element
`TableElement`	Accessible table structure element
`TableElement.CreateTHead`	Create the table header section
`TableElement.CreateTBody`	Create the table body section
`TableTRElement`	Table row element
`TableTRElement.CreateTD`	Add a data cell
`TableTRElement.CreateTH`	Add a header cell
`IllustrationElement`	Figure/illustration structure element
`IllustrationElement.SetImage`	Attach an image to the illustration

Logical Structure