PdfSecurity has properties that allow you to specify owner and user passwords for a C1PdfDocument. The OwnerPassword is required to change passwords and permissions. The UserPassword is required to open the document.

PdfSecurity also has properties that allow you to specify what permissions a regular user should have. For example, you may allow users to see the document but not to print or edit it.

You can specify permissions and set only the OwnerPassword, leaving the UserPassword empty. In this case, anyone will be allowed to open the document, but only the owner will be allowed to change the permissions.

Note that the encryption scheme used by Pdf is public and is not 100% secure. There are ways to crack encrypted Pdf documents. The security provided is adequate to protect your documents from most casual attacks, but if your data is truly sensitive you should not rely on Pdf encryption alone.