Display several files in one Preview window on Mac
You can set whether Preview opens multiple image files in separate windows or in the same window. You can also add files or pages to a PDF that’s already open.
Note: To open multiple PDFs in the same window, go to Apple menu 
. (You may need to scroll down.) Go to the Windows section, click the “Prefer tabs when opening documents” pop-up menu, then choose Always. See Change how folders open in the Finder on Mac.
Change settings for opening image files
In the Preview app
on your Mac, choose Preview > Settings, then click Images.Select an option for “When opening files”:
Open all files in one window: Uses one window for all files you open in Preview, no matter when you open them.
Open groups of files in the same window: Uses one window for each set of files you open in Preview at the same time.
Open each file in its own window: Uses one window for each file.
Add files to an open document
In the Preview app
on your Mac, choose View > Thumbnails.Drag the files to the window’s sidebar.