Failed to connect to Acrobat DDE server
- Latest Indesign with latest Acrobat running on Windows 10 (1709)
 - When exporting a PDF with activated option "Open PDF after export" and with Acrobat being already open with a document (but only in display mode and no dialogue window open), it would sometimes not open the PDF in Acrobat, but after a while show an error popup saying it failed to connect to Acrobat DDE server. I never had this in former versions of Indesign and Acrobat.
 
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Sophia Wilson
    
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  Read this blog onAcrobat Failed to Connect to a DDE Server, they have covered all the issues with their solutions on DDE Server I'll recommend you must read this post. It is very informative and I am sure your issue will be resolved.
Visit- https://www.adobesupportphonenumber.com/blog/troubleshoot-acrobat-dc-failed-to-connect-dde-server-windows-10/