Lists every layer — formally, "optional content group" — in a PDF, and whether each is visible by default. Common in CAD exports and redlined drawings.
Seeing what layers a CAD export or annotated PDF actually contains, including ones hidden by default.
Drop a PDF. Every layer appears immediately with its default visibility.
Most everyday PDFs (exported from a word processor, say) have no layers at all — this feature is mostly used by CAD/design tools and some scanned/redlined documents.