Represents the "F" entry of the thread action 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: 205, page 423.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 209, page 514.

The thread identifier is either an integer (zero-based index into the page's thread list) or a string (matched against the title in the thread's information dictionary). An indirect reference to the thread object is also accepted.