PDF layers (optional content) explained

Some PDFs let you toggle content on and off — a floor plan's electrical layer, or English vs French text. These are optional content groups, better known as layers.

What layers are for

• CAD and engineering drawings with separate systems.
• Maps with roads, terrain and labels on independent layers.
• Multilingual documents where each language is a layer.

How they behave

Layers are metadata that tell the reader which content to show. Hiding a layer does not delete it — the data is still in the file, which matters for privacy if a "hidden" layer holds sensitive notes.

Flattening removes them

Flattening a PDF merges visible layers into the page and discards the toggles. Do this before sharing if you never want a recipient to reveal a hidden layer.