Represents the "P" entry of the user properties attributes object.

It is a required entry defined as part of the PDF 1.0 specification.

It contains an array which contains UserPropertyElements.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 328, page 569.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 361, page 736.

The P entry holds an array of user property dictionaries, one for each custom property of the structure element. Each defines a single named property with its value, display string, and visibility flag.