Customer Portal Document Access

Customer Portal Document Access

Our application has documents within the database, however the users we have set up through the customer portal are unable to open these documents with their userid and pwds. When they click on the link for the documents they are asked to sign in again via a link to the url via zoho and NOT our landing page, but their credentials do NOT work, making the documents within our application un-accessible to them. How do we fix this issue?