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As the error message indicating that the application is already open, open the task manager and close all the background process running for Reader.
Then reboot the machine and then check if you can open the application or not.
If that does not help, end all the task for Reader in task manager and run the cleaner tool on the machine.
Restart your machine and then re-install the latest version from here Adobe - Adobe Acrobat Reader DC Distribution
Hope it helps.
If the issue still persists, please share the following details:
- OS version installed on the machine.
- Did you make any changes after which you start receiving the error message?
Let us know if you need any help.