Access Document Cloud file from Adobe Sign online

Whether I go directly to Adobe Sign, or start in my Document Cloud dashboard and select "Send for Signature", I can not access my Document Cloud files in Adobe Sign while on my computer (web browser).  I can only access Cloud files through Sign on a mobile device.  While on the computer, I can see the Library Documents, My Computer, Box, Dropbox, and Google Drive.  How can I access my Document Cloud files?  I guess this also begs the question, what is the difference between the Document Cloud and Library Documents?  Thanks.