You are creating an application that generates invoices in a variety of formats, including PDF,...

You are creating an application that generates invoices in a variety of formats, including PDF, ODS and HTML. Each of these formats is represented as a PHP class in your application. While some of the operations can be performed on all of the different formats (such as saving and loading), other operations may be specific to one or two of the formats (such as setting as read only). Which design pattern should you use for this application?