In between a network of Macs and an IBM System i sits Your Humble Narrator at a commodity Win10 box. And about every other PDF file I relay from Apple Valley to Big Blue generates this:
As a matter of policy, I blame everything possible on Adobe. But apparently the problem lies elsewhere:
On NTFS, files can have more than one stream of data. Usually there’s only one, the default, data stream.
Windows 2000 and XP used to use alternate streams to store additional properties which you could add in Summary tab of Properties dialog. This feature has been deprecated since Windows Vista. Yet you may still have files with such properties.
The warning is displayed when you copy a file which has alternate streams from NTFS drive to a drive that does not support alternate data streams. Therefore the alternate streams will be lost.
And Apple likes to pass on metadata through such streams; IBM scowls at such things. The best I can hope for is finding a toggle in Adobe Reader DC to shut this nonsense off. Unfortunately, I can’t find a damn thing in Adobe Reader DC.