Initializes a new instance of the PdfSignatureAppearance class.
public PdfSignatureAppearance(int pageIndex, PdfRectangle rectangle)
The zero-based index of the page the new signature field's widget goes on.
The widget's rectangle in the page's default user space; it must have a width and a height.
ISO 32000-2 §12.7.5.5 and §12.5.2 (Table 166, Rect and P).