With macOS High Sierra came Preview 10.0, thus taking away our ability to open multiple PDFs in one window (or a single tab, if you will).
This will not stand, man.
- Copy Preview.app with version 9.0 from a computer still running macOS Sierra, or from a Time Machine backup
- (Alternatively, checkout this repo or download https://github.com/frhd/macOS-Preview9.0/archive/master.zip)
- Move Preview.app to your User/Applications folder (not the one in the sidebar, but in your Home folder - create it if necessary)
- Set Preview.app (9.0) as default PDF Viewer for all PDFs (Instructions)
Of course, if you need Preview 9.0 for all users, rename it to Preview9.app and move it to the root Applications folder in the sidebar.