PDF (adobe.com)

About preserving visibility and locking

Objects and layers that are hidden in the PNG file are imported and remain hidden in Flash. However non-visible parts of a rich graphic symbol are not imported (for example, button Over or Down states).

If a layer is locked or hidden, all objects and sub-layers within the layer inherit the locked or hidden setting and retain it when they are imported into Flash.

If you import a single page into a new layer in Flash, one layer is created for the entire page and all objects are displayed. Visibility and locking attributes are not preserved.