Represents the "Length" entry of the encryption dictionary
It is an optional entry defined as part of the PDF 1.4
specification. It was deprecated in PDF 2.0.
It contains an integer representing a PDF numeric object.
The PDF specification states that this item assumes a value of
40 if no value has been provided.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 20,
Adobe Supplement to the ISO 32000, BaseVersion: 1.7,
ExtensionLevel: 3; Table: 3.18, page 14.
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 20, page 72.