PDFs that are shared in cloud services such as DropBox, Micrososft OneDrive, GoogleDrive, for example, have their own native PDF viewing apps and will force such files to open directly in their own supported file viewing apps.
Even if the Adobe Reader mobile app is set as the default PDF handler in you mobile device's, when you accesss PDFs in those cloud working spaces, the owner of such cloud workspace control how these PDFs will open natively.
They use their own PDF proprietary file viewers. That is the answer.
You may even notice that in some cases these PDFs won't even open in your default web broser if your web browser actually supports file viewing.
You will not have this problem, however, if you move the PDFs to Adobe Document Clound, which allows to share and view files in a similar convenient way, plus they will open in your device in the Reader mobile app natively.