2.1.26 [ISO32000-1] Section 12.3.3, Document Outline
The specification states:
A PDF document may contain a document outline that the conforming reader may display on the screen, allowing the user to navigate interactively from one part of the document to another. The outline consists of a tree-structured hierarchy of outline items (sometimes called bookmarks), which serve as a visual table of contents to display the document’s structure to the user.
Document outlines are not supported (their presence is ignored).