Hi Tim,
Thanks for your inquiry.
DocumentPrivilege is a class offered by Aspose.Pdf. You can specify rights you want to apply on a document (i.e you can only allow screen reading of the document). Basically you can differentiate multiple permissions by specifying privileges of a document. For example, a user can access a document which is not permitted to access but the user has privilege to access it.
While regarding Permissions, You can specify to which individuals or users you want to give access to the document and what they are allowed to do (i.e read, modify, control permissions). You can specify permissions by setting User Password and Owner Password.