In some older version of OS X, Preview.app used to open new PDF files as items in the sidebar. It still does this for images. For PDFs, they now only open as separate windows or as separate tabs within a given window. I'm currently running 10.14.2. The change apparently dates back to 10.13. I am having a hard time resurfacing the old behavior.
Here is a discussion from the apple forums on the same topic.
My preferred solution would be a
defaults write statement but something GUI would suffice. If this is not possible and an appropriate older version of Preview.app could be found, this could be an acceptable fix but I expect this solution may prove unstable in the long run.
My current settings are
preferences -> prefer tabs -> in full screen only and
Preview preferences -> images -> when opening files -> open groups in the same window.