PDF Bookmarks

In the PDF specification, what users commonly refer to as "Bookmarks" are formally known as the Document Outline.

It is not a static list of text; it is a complex, hierarchical tree structure constructed from Outline Item Dictionaries linked together as a doubly-linked list (each item points to its parent, its next sibling, and its first child).

An Outline Item doesn't just store a title string (like "Chapter 1"). Crucially, it stores an Action dictionary.

The most common type of action is a /GoTo action. This action contains a Destination array, which tells the PDF viewer exactly which internal Page Object to jump to when clicked. It can even specify exact (X, Y) coordinate points and a specific zoom magnification level.

When you edit or rearrange bookmarks using this tool's UI, we aren't just changing a text file.

Behind the scenes, we are traversing this doubly-linked list of dictionaries and surgically updating their internal pointers (First, Last, Next, Prev) and Action arrays in real-time before re-saving the file structure.