Represents the "F" entry of the submit-form action 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: 236, page 451.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 239, page 548.

The file specification holds the submission URL in its URL entry. The form data is sent to this URL using HTTP POST or, in some flag combinations, multipart/form-data or full PDF.