Represents the "FDF" entry of the catalog dictionary object.

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

It contains a FDFElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 28, page 73.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 3.25, page 23.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 29, page 97.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 242, page 458.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 245, page 555.

The FDF entry references an FDF (Forms Data Format) dictionary embedded within the PDF. This supports hybrid PDF/FDF documents that carry both the form definition and the form data in a single file.