Represents the "R" entry of the rectilinear measure dictionary object.

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

It contains a string representing a PDF string object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 262, page 481.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 267, page 594.