One annotation of a page an AnnotationLayer has loaded, by its place in the layer's page model.
public AnnotationTarget(int PageIndex, int Index)
The zero-based page.
The annotation's place in the page model: annotations in drawing order (the page's Annots order, widgets after the others),
leaving out popups and annotations that are never shown. Valid until the page's annotations change.
Spec #33 viewer capability 6; ISO 32000-2 §12.5.2.