Go to the install directory, i.e.” C:\Program Files (x86)\Adobe\Acrobat Reader DC\Reader\AcroApp\ENU”.
Create a new subfolder (I used “Disabled”).
Move 3 files from the “ENU” folder into the new “Disabled” folder: AppCenter_R.aapp & Home.aapp & Viewer.aapp.
Open a PDF and no more Tool Pane!
I originally moved just the “Viewer” file but if you clicked on “Home” or “Tools” on the toolbar you couldn’t go back to the “Document.” Moving all 3 files takes care of that issue. Like a lot of people I don’t and won’t ever use any of the tools. I just want a reader.