Think you need to check on permissions for customer sign-up to view their documents, navigate to settings > general setting > customer account

If you have chosen 'on invitation' then, you might have to send them a sign-up invite.
From the contacts app, select the contact whom you wish to give access then Action > Grant Portal Access and in the view use the checkbox to select the users you wish to provide access. (If your contact is a company, you would view all the individual contacts on the company in this view and can select multiple)

Post sign-up the customer would be able to view their documents.

Alternately, you can provide access from User administration screen by creating a new portal user