Set preview.app to default to "content only" view
I was looking for something that would seem to be the opposite of what you wanted (i.e. to have all PDFs open and show the "Thumbnails" of each page, as is an option in the "View" menu. 

As such, I found, on the Apple Support community the following: 

https://discussions.apple.com/thread/4582012

It would seem you have to go to Preferences> PDF> un-tick the "Always Show Sidebar" option beside "Opening for the first time" option. 

Hope that helps a bit! (?)