I can find nowhere to set Preview to default to Thumbnail View with all PDFs. I use them every day. I have ticked Thumbnails in the View menu but it does nothing. Does anyone know how to set the default?
Have you tried setting Preview to always open with the sidebar? You can do this in Preview menu > Preferences > PDF tab and selecting the checkbox for "Always show sidebar".
Right click, select Get info, select open with, set default