Represents the "Unix" entry of the full file specification
It is an optional entry defined as part of the PDF 1.0
specification. It was marked obsolescent in PDF 1.7 and deprecated
in PDF 2.0.
It contains a string representing a PDF string object.
This string contains raw byte data. So it looks like a string
but really it is just a wrapper for data.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 44,
Adobe Supplement to the ISO 32000, BaseVersion: 1.7,
ExtensionLevel: 3; Table: 3.41, page 26.
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 43, page 133.