Represents the "Thumb" entry of the page object object.

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

It contains an ImageXObjectElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 30, page 78.

Adobe Supplement to the ISO 32000, BaseVersion: 1.7, ExtensionLevel: 3; Table: 3.27, page 23.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 31, page 104.

The Thumb entry references a thumbnail image stream providing a low-resolution preview of the page. Viewers may use this to display a page thumbnail without rendering the full page content.