All the documentation is available here. Acrobat DC SDK Documentation
The DDE specific part is under the Acrobat Inter-Application Communication section, this link opens the actual documentation- Acrobat DC SDK Documentation
Although please note the documentation is not as user friendly as it could be, once the page opens you may have to open the bookmaarks ( 3 bullet point icon), and select "Acrobat Interapplication Communication" Then "Interapplication Communication Reference" Then " DDE Messages"
Hope this helps