Represents the "FD" entry of the font descriptor object.

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

It contains an Element.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 122, page 281.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 120, page 341.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 124, page 285.

FD maps font dictionary names to partial font descriptors that override selected entries of this one. It is used in CIDFont descriptors to associate per-glyph-range metrics with subsets of the font's glyph set.

Each entry in the FD dictionary is itself a font descriptor dictionary containing only the entries that differ from the top-level descriptor.