In the PDF specification, Optional Content is managed through several interrelated objects:

<strong>Optional Content Group (OCG)</strong> — A dictionary with a /Name entry that defines a single layer. The OCG has a state (on or off) that controls whether its associated content is visible.

<strong>Optional Content Membership Dictionary (OCMD)</strong> — Groups multiple OCGs with a logical operator (Any, All, None, Not-Any) to define visibility based on the combined state of multiple groups.

<strong>Optional Content Properties Dictionary (/OCProperties)</strong> — A document-level dictionary that lists all OCGs, defines the default On/Off states, and specifies named configurations (sets of OCG states that can be applied at once).

<strong>Usage dictionary</strong> — Each OCG can have a Usage sub-dictionary specifying when the layer should be on or off based on context: print (<code>/Print</code>), screen view (<code>/View</code>), zoom level (<code>/Zoom</code>), export (<code>/Export</code>), or user language (<code>/Language</code>). A layer with a /Print usage set to off will be visible on screen but not printed — a common approach for screen-only watermarks or annotations.

Content is associated with an OCG by wrapping it in a marked content sequence using <code>BDC</code> (Begin Marked Content) with /OC and the OCG reference as the tag. Any content on a PDF page — text, images, paths, annotations — can be made part of a layer.

<strong>CAD and architectural drawings</strong> — Engineering drawings exported from <a href="https://www.autodesk.com/products/autocad/" target="_blank" rel="noopener">AutoCAD</a>, <a href="https://www.vectorworks.net/" target="_blank" rel="noopener">Vectorworks</a>, or <a href="https://www.bentley.com/" target="_blank" rel="noopener">MicroStation</a> to PDF typically preserve the original drawing layers as PDF optional content. Contractors can turn specific layers on/off (electrical, plumbing, structural) for review.

<strong>Multi-language documents</strong> — A single PDF with content in English, French, and German on separate layers, each configured with a /Language usage entry. Viewers with language detection can automatically show the appropriate layer.

<strong>Print vs screen variants</strong> — A PDF with a high-resolution version on a print layer and a lower-resolution version on a screen layer. Or navigation elements (clickable buttons, hoverable regions) on a screen-only layer that should not appear in print.

<strong>Step-by-step instructions</strong> — Educational or training PDFs where each step's content is on a separate layer; the viewer can reveal steps progressively.

<strong>Version comparison</strong> — Documents with "proposed" and "current" content on separate layers for markup review.

<strong>Map layers</strong> — Geographic maps with separate layers for roads, points of interest, administrative boundaries, and terrain — allowing users to customise the map view.

Layer support varies across viewers:

<strong>Adobe Acrobat / Adobe Reader</strong> — Full layer support. The Layers panel (View > Show/Hide > Navigation Panes > Layers) displays all OCGs with toggle controls. Usage dictionaries are respected (print vs screen, language, zoom).

<strong>macOS Preview</strong> — Shows all layer content simultaneously (renders all OCGs as on) without a layer panel. Layers cannot be toggled.

<strong>Chrome / Firefox (PDF.js)</strong> — Partial support. <a href="https://mozilla.github.io/pdf.js/" target="_blank" rel="noopener">PDF.js</a> renders layers but the layer panel toggle UI varies by version. Usage dictionaries may not be fully respected.

<strong>Foxit Reader</strong> — Full layer support comparable to Adobe Reader.

For PDFs where layer control is important — CAD drawings, multi-language documents — recipients should use Adobe Reader or Foxit. For PDFs where layers are used for print/screen variants, test in the target viewer to verify the correct layer is active for each context.

<strong>Printing considerations</strong> — Layers with /Print OFF usage entries will not print even if they are visible on screen. This is intentional for screen-only annotations. If a PDF appears to print with missing content, check the Layers panel in Acrobat for layers set to not print.

PDF Layers interact with accessibility in two important ways:

<strong>Reading order</strong> — Screen readers read the PDF content stream in order. If multiple language layers are present and all are rendered simultaneously (because the viewer does not support layers), the screen reader will read all language versions of every text block. This produces a confusing and inaccessible result. For multilingual PDFs that must be accessible, separate PDF files per language is more robust than language layers.

<strong>Flattening layers</strong> — For archiving and accessibility compliance, <a href="https://pdftheguide.com/glossary/flattening" target="_blank" rel="noopener">flattening</a> a PDF merges all visible layers into the base content and removes the optional content structure. This ensures consistent rendering across all viewers and removes the dependency on layer support. For <a href="https://pdftheguide.com/glossary/pdf-a" target="_blank" rel="noopener">PDF/A-1 and PDF/A-2</a> compliance, optional content is technically permitted but all states must be shown simultaneously in the default rendering — a restriction that effectively means layers are rarely used in PDF/A archival documents.