Represents the "3DD" entry of the 3d reference dictionary object.

It is a required entry defined as part of the PDF 1.0 specification.

It contains a D3DStreamElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 303, page 519.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 314, page 648.

The 3DD entry holds an indirect reference to the 3D stream dictionary containing the model data. This reference links the external data dictionary to the actual 3D content.