Represents the "ID" entry of the folder dictionary object.

It is a required entry defined as part of the PDF 1.7 Extension Level 3 specification.

It contains an integer representing a PDF numeric object.

It should have a minimum value of 0.

For definitive details see:.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 8.6c, page 32.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 159, page 451.