Adobe - Open New Window Instead of Tabs
Follow the below steps to have Adobe PDFs open in new windows instead of in tabs.
Open Adobe (Reader, Pro, Basic) and click on the Edit menu then Preferences.
Remove the check from the box next to Open documents as new tabs in the same window (requires relaunch).
Close and Reopen Adobe
Close out Adobe and reopen it. Now when you open multiple PDF's it will open them in new windows instead of tabs in the same window.