The navigation pane is an area of the work space that can display different navigation panels. Typically, these panels act like a table of contents, with items you can click to jump to a specific place in the document. For example, the Pages panel contains thumbnail images of each page; clicking a thumbnail opens that page in the document.
When you open a PDF, the navigation pane is closed by default, but buttons along the left side of the work area provide easy access to various panels, such as the Pages panel button and the Bookmarks panel button . When Reader is open but empty (no PDF is open), the navigation pane is unavailable.
Click any panel button on the left side of the work area to open that panel.
Choose View > Navigation Panels > Show Navigation Pane.
Click the button for the currently open panel in the navigation pane.
Choose View > Navigation Panels > Hide Navigation Pane.