Corporation Tax > Select Client > Settings > Data Security
Help Guide
Password Protection has been added to Documents within the Corporation Tax module and can be enabled by heading to the Navigation above 
Once there click the edit button which will allow you to Enable the document protection for the first time. Once enabled, you can then enable password protection for the following reports; CT600 PDF & Computation PDF.
Once enabled, use the Master Password section to setup the password which is unique for each client.