Hope you are doing well and sorry for the delay in response. I hope you would be able to resolve the issue by now, if it still persists, we would need some more information for a better understanding:
- Is this a behavior with a particular PDF file or with all the PDFs that you try to sign? Please try with a different PDF file and check
- What is the version of the Adobe Acrobat Reader DC 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.
- Are you on Mac or Windows Machine and what is the version of the OS? Please check for any missing/pending update for OS and try updating it and check.