I remember having a sidepane with the table of contents in a specific PDF file. I played around with the program and now it is closed. I can't find a way to reopen the above mentioned sidepane. Is there a hotkey for that or what is the name of the option?
With 'Document Viewer' I refer to the Ubuntu standard pdf application.
As described by '@guillermo chamorro' in the comments: when I press F9 the sidepanel pops up. It can show different things, that are selectable in the dropdown menue on top. To view the table of contents of a document that drop down needs to be set to the 'Outline' setting.