As pointed by jerryANDtom at https://forums.adobe.com/thread/1810972

"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!"

Man, it worked like a charm!

There is a workaround that may work. It involves removing some files (adobe applets) to another directory.

User jerryANDtom pointed it out in a thread on the Adobe forum:

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 three files from the ENU folder into the new Disabled folder: AppCenter_R.aapp, Home.aapp and Viewer.aapp.
Open a PDF and no more Tool Pane!