Represents the "Unix" entry of the embedded file set dictionary
It is an optional entry defined as part of the PDF 1.3
specification. It was marked obsolescent in PDF 1.7 and deprecated
in PDF 2.0.
It contains an EmbeddedFileStreamElement.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; page 103.