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

It is an optional entry defined as part of the PDF 1.0 specification.

It contains an integer representing a PDF numeric object.

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 ParentTreeNextKey entry holds the next available integer key for insertion into the parent tree. When a new page is added and assigned a StructParent key, the new key must not exceed this value, which is then incremented.