Gets the presentation order of the groups for a user interface (Order), as a tree.
public IReadOnlyList<PdfOptionalContentOrderItem> Order { get; }
ISO 32000-2 §8.11.4.3, Table 99. Absent: empty in the default configuration, the default configuration's order in the others. Nesting deeper than 32 levels is cut off, and an array reached a second time (a cycle or a shared array) is read once.