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

It is a required 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: 97, page 221.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 95, page 272.

The F entry holds a file specification identifying the external PDF file that contains the referenced page. The referenced file must be accessible at output time for the reference to be resolved.