Adds an annotation to a loaded page through the annotation API (for example
layer.Add(0, page => page.AddSquareAnnotation(rect, options))), redraws its area and selects it.
public AnnotationTarget? Add(int pageIndex, Func<PdfPage, PdfAnnotation> create)
The zero-based page.
Creates the annotation on the page it is given.
null when it does not take part (a popup, or hidden).ISO 32000-2 §12.5; spec #33 viewer capability 6.