Shrinking window by itself

Hello everyone,

I have a recurring problem with Adobe Reader.
I am on Mac and when I open a PDF file with Adobe Reader and put it in full screen with the green icon at the top left of the window, there are no problems. But as soon as I stay a little too long on another window, usually Firefox or any browser ; my Adobe Reader window is shrinking by itself and I have to reclick on the icon in the Dock to make the window reappear. It’s not even on the Desktop anymore.

How can I deal with this problem?