The Outline pane can include two icons to help you navigate forward and backward through the outline. As you use these buttons to navigate, the outline expands as necessary to display the next document or documents depending on the document type. These navigation icons only appear if the publisher of the content you are viewing made them available. If these controls do not appear, you can navigate documents in the outline by clicking on them.
Note about outline bookmarks: When you exit and restart the Player, it can automatically display the last document you were reviewing in the outline. These document bookmarks are saved only if your content provider has published the Player content you are viewing with this bookmarking capability.
and backward 
through the outline. As you use these buttons to navigate, the outline expands as necessary to display the next document or documents depending on the document type. These navigation icons only appear if the publisher of the content you are viewing made them available. If these controls do not appear, you can navigate documents in the outline by clicking on them.