Would you know if the customer contact has permissions to view their documents on the portal view?

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 of 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