Acrobat Reader app especially, free uses for an open pdf file, which in server transfer to the data for transformation in a particular format. However, when we try multiple documents as text, images at one time, then it shows an error as a prompt acrobat failed to connect to dde server . Yet, it does frustrate you, although, then you can take advice to Digi Knowloogy to solve this issue and for more information.
https://digiknowlogy.com/blog/how-to-fix-the-acrobat-failed-to-connect-to-dde-server-error/