Tab order through the document: the viewer itself, then its links and focusable controls in reading order, then whatever follows the viewer. Shift+Tab walks the same stops backwards, and entering the document backwards lands on its last stop. After a jump to an anchor, Tab from the viewer goes on from that anchor instead of from the top of the document.
(bool handled, IInputElement? next) ICustomKeyboardNavigation.GetNext(IInputElement element, NavigationDirection direction)