Each page has 4 canvasses: an underlay, a visual underlay, an overlay and a visual overlay, all of type Canvas. You draw underneath or on top of a page by adding shapes to one of the 4 layer objects. The layers are non-creatable and retrieved through the [Visual]Underlay and [Visual]Overlay properties of a page object.
The underlay and overlay uses the original coordinate system and doesn't respect page orientation and/or any clipping area. The visual underlay and visual overlay uses a coordinate system that does respect page orientation and/or any clipping area. The visual layer will be placed exact on the resulting visual area shown in a viewer (e.g. Acrobat Reader). Note that if no rotation and/or clipping is available that the visual layer has the same result as the normal layer.
public class Canvas : Object,IEnumerable<Shape>