Hi Vlkram,
If you do not want to change the business process, I recommend Fiori Approve Requests app. Attachments are stored in workflow container.
Approvers can approve/reject/forward/comment but there is no creating attachment in the standard.
If creating attachment from approver side is mandatory, you can extend the standard app by using UI extension points. Please look at SAP Fiori UX - Extensibility.
Is there any business requirement for approving documents in their mobile devices? In that case, is excel attachment the best option?
Regards, Masa
SAP Customer Experience Group - CEG