Represents the "F" entry of the fdf dictionary object.

It is an optional entry defined as part of the PDF 1.0 specification.

It contains a FileSpecificationElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 243, page 459.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 246, page 556.

The file specification in F identifies the PDF document that this FDF data applies to. When importing, the viewer uses this to verify it is applying data to the correct document.