Obsolete Solution in Acrobat Failed to Connect to DDE Server
As we look at some documents these days, we try to keep securely in from one place to another place. In which sometimes, we don't remember and if we want to do some additional modification in documents, whereas waste too much time for this procedure. So the Digi knowlogy suggest you, Acrobat reader for more conventional use, which through easily you can manage your document. But if you send multiple files, then you can get prompt Acrobat Failed to Connect to DDE Server.
https://digiknowlogy.com/blog/how-to-fix-the-acrobat-failed-to-connect-to-dde-server-error/