Represents the "ParentTree" entry of the structure tree root object.

It is defined as part of the PDF 1.0 specification.

It contains a NumberTreeNodeElement containing Elements.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 322, page 557.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 354, page 720.

The ParentTree entry references the root of a number tree mapping integer keys to structure elements. Every page with tagged content has an entry here, enabling a reader to find the structure elements appearing on that page.