Open files in background on MacOS Sierra?
If you just want to go back to Finder you just need to open Script Editor and paste this code:



repeat
    if "Finder" is not frontmost application then
        tell application "Finder"
            activate
        end tell
    end if
end repeat



Save it as an application, like FinderAlwaysON and execute it before starting your work. 



Be aware that this will lock your computer with Finder, so when your are done opening the files bring up Force Quit (Command+Option+ESC) and close the app.