Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, provide a broker for client applications. In one aspect, a method includes providing an electronic document that has a document format compatible with a document reader application. The electronic document includes a broker application and a client application. The broker application can be loaded into a virtual machine running in the document reader application on a client device. The loaded broker application loads the client application into the virtual machine, and the loaded client application provides a graphical user interface for display. In some instances, the loaded broker application receives data from the loaded client application and stores the received data as a data set in the electronic document. In some instances, the loaded broker application retrieves a data set from the electronic document and provides the retrieved data set to the loaded client application.