At some time, when we open a file in the pdf document to transfer a dynamic data exchange between applications on the window to manage the device, and peculiarity. It displays the error acrobat failed to connect to a DDE server. Mostly it happens when the tool is enormously involved with another file or in hang state.
https://digiknowlogy.com/blog/how-to-fix-the-acrobat-failed-to-connect-to-dde-server-error/