`ifAvailableShowOutlineOnLoad` looks like a copy-paste error :-)
More to the point though, with the introduction of a preference for the attachments view, we will have two mutually exclusive preferences. This will be confusing in PDF files which include both an outline and attachments.

I would suggest removing the pref for now. Once this PR lands I would be happy to follow-up with a patch that introduces a preference, that enables the user to choose which sidebarView to opened on load (i.e. either "thumbs", "outline" or "attachments").