Gets the 0-based index of Page in Pages, or null.
public int? PageIndex { get; }
ISO 32000-2 §12.3.2.2. A remote destination's page number is never resolved in this document. A local destination whose first element is an integer uses it as the index, with a diagnostic when the destination is read. A structure destination (§12.3.2.3) uses the page of the first marked-content or object reference among its element's kids, processed in array order and descending into child elements; when none identifies a page, the first page.