To hide the Tools pane in Adobe Reader DC 2019.008.20071 or 2019.008.20074, open the registry and navigate to the following registry key:

HKEY_CURRENT_USER\Software\Adobe\Acrobat Reader\DC\AVGeneral

Here, create a new String Value called “aDefaultRHPViewModeL”. Set its value to “Collapsed”.

When you start Adobe Reader DC after creating that String Value, you should never get the Tools pane anymore.

I haven’t checked if you could create the same String Value in HKEY_LOCAL_MACHINE.