Using activation keys for Adobe acrobat DC

Of course, a seller saying a code is legitimate doesn't make it so. In fact, such a phrase strongly suggests a fake. 

Hi there

++ Adding to the discussion

What happens when you try to use the serial number? do you get the error message for revoked or invalid serial number? If yes, please check the help page https://helpx.adobe.com/x-productkb/global/invalid-revoked-serial-numbers.html

Regards

Amal

Acrobat DC doesn't have any "activation keys", which makes me suspect you probably purchased an illegitimate volume license, or something like that.

Adobe does sell through third-parties, but they have to be recognized resellers. Otherwise you're very likely to get pirated software, if you're lucky, and all kind of viruses or malware, if you're not.