Your clients can attach documents to a message by following these instructions:
Tap the blue plus sign (+) at the bottom of their screen.
Select Compose a Message.
Tap the To field to add a recipient and select an Account.
Enter a Subject for the message.
Write their message by tapping inside the Write a Message... box.
Tap Add Attachment.
Choose from Photo Library, Scan Document, or Import File from....
Rename the file if desired.
Optionally tap Add Tags & Year to further categorize the file.
Tap Apply.
Tap Upload.
Finish by tapping Send in the upper right-hand corner of their screen.