Represents the "First" entry of the object stream dictionary object.

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

It contains an integer representing a PDF numeric object.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 16, page 46.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 16, page 61.