There is currently a bug which prevents documents stored with Specific People the ability to copied (JIVE-59577). The work-around is to store the document in a private group. I see that your Community manager has filed a case around this in your support group and they will be notified when the bug is resolved.
If anyone else has this issue and would like to be alerted as soon as the bug is resolved, i’d encourage anyone to file a new case so we can associate the bug ID to your case. we can then reach out once the bug is fixed.