This for the Pro DC application...not the Reader.
This happens even when the app is already running with another pdf file opened and the Tools panel is hidden.
Supposedly Adobe fixed the issue, and below is the Acrobat paragraph of an Adobe forum document created by Sara.Forsberg in July 2015 stating (with my observation in red):
Acrobat
There's nothing special that you need to do. Acrobat will remember the state of the Tools panel from one session to the next. (This is not my experience! Each time I close the app with the panel hidden (closed), it opens up every time a PDF is opened. It even happens when the app is already running with another PDF file opened, but the Tools panel is hidden.) Opened or closed, you choose! (You can toggle the Tools panel off by clicking the small arrow on its left border.)
I've looked under Preferences to see if there's something to check/uncheck, but can't find anything that would take care this issue.
AdChoices