Okay, seems I've stumbled upon an ugly workaround but it works for me. I'm using Windows 8.1. 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. Let me know if this works for you.
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