Is this a behavior with a particular PDF file or with all the PDFs that you try to open? Please try with a different PDF file and check.
Are you on Mac or Windows OS and what is its version?
What is the version of the Adobe Acrobat Reader you are using? To check the version go to Help > About Acrobat and make sure you have the latest version installed. Go to Help > Check for updates and reboot the computer once.
Also, please share the screenshot or a small video of the issue for a better understanding.