PDFs open in tab not new window

I have made these changes to the Dock settings and still no luck. Just an update, it is now happening randomly. For example, when opening multiple PDF attachments (one at a time) from a mail message, they each open in their own window. However when opening pdf files from client folders, sometimes they open in their own windows, others time they open in tabs.

Another update, it seems to be fixed (hopefully it stays that way). I had the dock preference for tabs set to manually, I then switched it to "in full screen only" with no luck. Once I switch it back to manually it now seems to be working, opened PDF regardless of location are opening in their own window. This seemed to work for someone else in an older thread as well. FWIW, I also unchecked the box in Finder > Preferences > General next to "open folders in tabs instead of windows".

Thanks for everyone's time.

Well it's a new day. I knew I spoke too soon. I changed zero settings since it was "fixed" yesterday and Preview is back at it. I'm convinced this is a glitch with High Sierra at this point and I'm calling Apple now.

I'm using preview primarily but I changed this setting in Acrobat anyways. Unfortunately, no luck.