We are sorry for the trouble and the delay in response.
Is this a behavior with a particular PDF file or with all the PDFs that you open? Please try to open a different PDF file and check. If its a file specific issue please share the file with us for testing. Upload the file to the document cloud (https://documentcloud.adobe.com/link/home/) generate the link and share the link with us.
Make sure you have the latest version 20.006.20034 go to Help > Check for Updates.
Also, please check for any necessary and recommended updates for the video drivers and firmware and try removing them and reinstalling and see if that helps.
Thanks for your suggestions. I checked my Adobe Acrobat Reader DC version and it's the latest, 20.006.20034. I've also checked the (Radeon) video driver version and it is the latest too.
I get this behaviour on most, though not 100%, of PDFs, including those I generate and many received or downloaded from various places on the internet.
We are sorry to hear that the issue still persists. lease try the following steps and see if that helps:
Go to Adobe Acrobat / Reader DC (Mac), Edit (Windows) > Preferences > Enhanced security > Turn Off the Protected mode and uncheck the enhanced security > Click OK and check.
Note: Please turn on the security after testing.
If it still doesn't work, try to create a new test user profile with full admin rights or enable the root account in Mac and try opening the PDF there and check,