I noticed that when I have a PDF file already open in Acrobat Pro DC, for instance opened it from a local disk, and then want to open a PDF attachment in Microsoft Outlook, it fails with the "Acrobat failed to connect to a DDE server." error message.
However, if I just close the PDF that is open and then try to open the PDF attachment of an email in Outlook it opens with no problem. Similarly, if I already have an PDF attachment open from Outlook (not a PDF instance from local disk, e.g.,) then when I try to open another PDF attachment in Outlook, it can open again with no problem.
The opposite case, fortunately, does not cause errors. I.e., if I first open a PDF attachment in Outlook and then try to open PDF files from local hard disk, it opens them successfully.
Btw, I noticed it is the same case when I open a PDF file from my browser while already having one from local hard disk open!
In short, it seems if I already have open PDF files from one source (e.g., hard disk), and then try to open a PDF attachment from another source such as Outlook or Firefox, it fails.
How is it possible to fix it? It is so frustrating that I have to close all my working PDF documents just to be able to view a PDF attachment.
Sorry for the delay in response to your query and apologize for the inconvenience caused. As per the description above, you are not able to open a PDF file attachment in Outlook when a PDF file is already opened in Acrobat, Is that correct?
Please check for the latest update of Acrobat Pro from Help>Check for updates. You can also download the updates manually from Overview — Acrobat and Adobe Reader Release Notes
You may also try to repair the installation files of Acrobat Pro from Help>Repair installation(only for Windows).
Reboot the machine once after the update and repair of Acrobat. And see if this brings any difference.
You may also try to repair the Office/Outlook once.
If the issue persists, please share the following details:
Let us know how it goes.