Startup window should open at Startup AND when all PDFs are closed
It's good for those of us who utilize Document Cloud and Creative Cloud to have the startup window with access to these files. However, if you close all of the PDFs you have open, this window is no where to be found unless you quit and restart the application (unless it's hiding somewhere in a menu, but why would that be?).
If you wish to see the Home screen even after closing all the pdfs, that is how you can do it.
1) launch Acrobat
2) go to preferences (Cmnd+K)
3) General -> select "Show Home screen when all documents are closed"
4) Click OK
Now when you will close documents, Acrobat will still be launched for you will the available options like creative cloud ,Document cloud etc.
Let me know for any other asks.