The copy: for a reference, the reference to the target's copy (the same reference every time the session copies it); for a
direct container, a new container to place in the target; an immutable value itself. Instance for a
reference to nothing, to a page tree node, or to the source's encryption dictionary.
Remarks
ISO 32000-2 §7.3.10. A page (a dictionary whose Type is Page) is copied as CopyPage(PdfPage) copies one,
inheriting through its Parent chain; every other page the graph reaches is handled as the class remarks say. A mutation
of the target.