Major Description
Current implementation of PDPropBuildDataDict does not support /REx (application implementation version) entry.
The /OS entry also does not conform to the latest PDF Signature Build Dictionary Specification from Adobe, which specifies /OS entry value type as array. PDF Signature Build Dictionary Specification v1.4, that is referenced from ISO 32000-1, also specifies this entry as array. The example provided in Specifications indicate /OS entry type as array of name objects.