You can determine which users are able to view the "Documents" tab in the customer profile by enabling this option in the user setup by following these steps:
Go to Setup > Users and click on the company link
Select the company user that you want to set this option for.
Under the Actions column click on Edit.
Go to the Options tab.
If you want to enable the access to the documents tab, check the View Customer Documents option. If you want to keep the user from viewing this information leave the option unchecked.