When you try to install Acrobat DC from Creative Cloud desktop application, you get an error - “Another installer is currently running, which must be closed before installing the product. Please close other installers. (Error code: 81)
Close all the Adobe processes and restart your computer. Try installing Adobe Acrobat DC.
For more information, see Close conflicting processes or applications
The Adobe Application Manager or the PDApp.exe is installed in the PDApp folder. When you try to install or update any Adobe application from Creative Cloud desktop application, it invokes this process to run in the background. Renaming the folder will force close the process running in the background.
If the PDAPP folder is not visible in the specified location, change your system settings to allow hidden files and folders to be displayed. To enable the setting, see the below articles: