Represents the "W" entry of the cross-reference stream
It is a required entry defined as part of the PDF 1.0
It contains an array which contains integers, representing PDF
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 17,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 17, page 66.