Represents the "S" entry of the collection sort dictionary
It is a required entry defined as part of the PDF 1.0
It contains an array which contains strings, representing PDF
If you read the specification you will see that an array of one
string may be represented as a single string rather than an array.
However we always present this property as an array to simplify the
interface. It is only if you add or remove items in the array, that
the underlying representation will be changed.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 158,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 156, page 449.