The 'In a new topic pane' invocation option causes the application plug-in or add-on related to the file (the Acrobat Reader in this example) to add a new instance of the
topic pane.[The topic pane is the area in your browser that contains a topic: the content, the title and eventually the action bar. If you browse a book or a folder then the topic you select in the folder tree is shown on the right in the topic pane.]

Invocation option 'In a new topic pane'
You redisplayed the topic by clicking on the 'Back' navigation item on top of the topic pane as indicated by the red arrow in the next image.

Application file shown in a new topic pane