PDF layers for PDFium

In the layers panel (sidebar → layers tab)

• Uncheck a layer — the drawing repaints without it. Nothing is written to the file; visibility is view state.
• Tick “Labels (French)” — English switches itself off. The document declares them a radio pair, and the panel honours it.
• Click a row — it becomes the current layer: where new annotations go, and what Delete acts on.
• Drag rows — reordering rewrites the document’s own /Order tree; drag a heading and its children move with it.

With annotations

• Draw with a layer current (Annotate or Shapes tab) — the mark lands on that layer and hides with it.
• Select an annotation, press its layers button — a dropdown moves it to any layer, or off layers entirely.