Gets the destination shown when the document is opened (OpenAction as a destination array), or null
when the entry is absent or is an action (see OpenAction): the top of the first page.
public PdfDestination? OpenDestination { get; }
ISO 32000-2 §7.7.2, Table 29, and §12.3.2. The entry shall be an array or a dictionary; a name or string is read as a named
destination with an OpenActionInvalid diagnostic, and any other value as absent with the same diagnostic.