What Is Nesting in PDF Accessibility and Why It Is Important?

Nesting in PDF Accessibility

Nesting in PDFs 101: A Beginner's Guide to Accessible Document Creation

Opening a PDF document might seem simple for most people. But for those who use screen readers to access information, how the document is organized internally becomes very important. This organization of the internal structure, determined by how electronic tags are arranged within the PDF, is called nesting. Nesting needs to be done correctly so screen readers can understand the layout, different parts of the document, and ultimately, what it means. By making sure the nesting is logical, we can create PDFs that are accessible and make it easier for people using screen readers to find the information they need.

Why Nesting Matters?

Using proper nesting is an important part of creating PDFs that are easy for everyone to use. Here’s how nesting helps:
  • Streamlined Navigation: Screen readers rely on the document's underlying structure, defined by tags, to guide users. Correct nesting creates a logical flow, allowing users to easily jump between headings, tables, and figures.
  • Contextual Clarity: Assistive technologies take nesting a step further, using it to understand the relationships between different elements. Think of a figure nested within a list. Proper nesting clarifies that the figure is an integral part of the list's explanation, not just a random image.
  • Enhanced User Experience: A well-nested PDF goes beyond just accessibility. It offers a smooth and predictable experience for all users, regardless of their abilities. This ensures everyone can focus on the content itself, not the struggle to navigate it.

What is Tag Nesting and How Does It Impact PDF Accessibility?

PDF tags are like hidden organizers that control how a tagged PDF document is understood by assistive technology.

When tags aren’t accurate or are in the wrong order, it can make reading the document confusing or even impossible.

Sometimes, it’s up to the person fixing the document to decide the order and organization of these tags. But other times, the tags need to follow specific rules set by PDF accessibility standards. Not following these rules precisely can result in a document that doesn’t meet accessibility requirements.

When it comes to nesting tags, it’s crucial to make sure they’re in the right place. For example, if you have an image in a list item, the Figure tag should be nested inside the List’s structure. This nesting gives important context to the reader, showing that the image is part of the list.
Placement value is another aspect of nesting to consider. It determines whether a tag can have other tags inside it or if it needs to be nested within other “structural” tags. While placement value can sometimes be changed, it’s essential to understand that it affects how tags are nested within the document.

How Nesting of PDF Help for Complex Documents:

  • Modular Content for Easier Updates: Frequently updated sections can be transformed into nested PDFs within the main document. This allows for targeted revisions without needing to modify the entire file, making it much easier to keep the content up-to-date while maintaining accessibility features.
  • Potential for Smaller File Sizes: By nesting less frequently accessed content, the overall size of the main PDF could be reduced. This translates to quicker loading times and a smoother experience for users with slower internet connections.
  • Improved Navigation: Imagine a central PDF that acts like a master document, with each chapter existing as a separate, embedded PDF. This would allow users, including those who rely on screen readers for accessibility, to easily jump between sections using clear and well-organized navigation tools within the main document. The experience would be similar to how everyday users navigate a website with its various subpages.

Why to Perform Tag Nesting for PDF Accessibility?

Understanding tag nesting makes its purpose clearer. Simply put, some tags need specific nested structures to be accessible.
For instance, when tagging a list in a document, it must be correctly marked as a List. List tags should include LI (List Item) tags for each list item. If a list has 15 items, the List tag must have 15 nested LI tags.
These LI tags also require substructures for better organization, typically LBody (body) tags, and Lbl (label) tags. The Lbl and LBody tags provide crucial information about the list’s composition, order, and numbering system.
Some lists contain other lists within them, known as “nested lists,” which follow the same structural rules. Tables and Tables of Contents are other examples needing nested tags.
Nesting can also organize content using “Grouping” tags, such as Document, Part, Sect, and Art tags. Some, like the Document tag, are mandatory and contain the entire Tag tree. Others, like the Art tag, may be optional or context-specific.
An Art tag might group a newspaper article spanning multiple pages, helping readers navigate the entire article easily. Grouping tags like Art have their own rules; for example, the Art tag cannot directly contain text. Instead, it nests heading tags, paragraph tags, and others as needed.
In summary, nesting is essential for using tags correctly, whether for lists, organizational purposes, or other tag structures.

Challenges for Making Nesting PDF Accessible:

Making nested PDFs accessible can be tricky for a few reasons,
  1. Limited software support: Not many programs can currently create nested PDFs easily. However, some advanced tools are starting to explore this feature.
  2. Accessibility guidelines matter: Even if nested PDFs become more common, they'll need to follow WCAG guidelines to work with screen readers. This involves:
    • Proper tagging: Information inside nested PDFs needs clear labels using WCAG techniques. This helps screen readers understand how things are connected and present them in a logical order.
    • Clear organization: The way nested PDFs are organized should be consistent and easy to follow. Screen readers rely on a well-defined structure to deliver information clearly to users.

How to Add Nesting Tags in PDFs Using PREP

The way you nest tags inside each other can vary depending on the software you’re using. Some programs might have different buttons or shortcuts for doing this.
It’s generally easier to nest tags in some programs than others. But the most important thing is to know which tags can (and can’t) be nested together for the best results.
Even if you understand nesting, there are some common mistakes to watch out for. One is nesting too many tags of the same type. For example, some software might allow you to nest multiple “Section” tags inside each other. While this might not affect how the document is read, it’s an easy fix you can make when working on the file.
Also, it’s important not to nest the same type of tag directly inside itself. For instance, a “Paragraph” tag shouldn’t have another “Paragraph” tag nested within it.
There are some situations where nesting rules change depending on the context. For example, a “Caption” tag can be used for both figures and tables.
  • A caption for a figure shouldn't be nested inside the "Figure" tag itself, because the caption wouldn't be read aloud by screen readers.
  • However, a caption for a table should be nested inside the "Table" tag to ensure it's read with the table content.

These situations might seem specific, and they are. But with practice, you’ll be able to recognize them easily as you work on your documents. If you’d like to learn about how a PDF tagging software like PREP can help you with nesting, feel free to contact us.

Closing Thoughts

Nesting is a critical but sometimes challenging aspect of PDF accessibility. By understanding its importance and following all the accessibility guidelines, we can create PDFs that are not just accessible but also improve the overall user experience for everyone. With advancements in software and a continued focus on best practices, nested PDFs have the potential to become a powerful tool for delivering information in a clear and organized way.

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Debangku Sarma

Digital Marketing Associate
Continual Engine

Vijayshree Vethantham

Senior Vice-President, Growth & Strategy
Continual Engine US LLC

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