Multimedia documents are of importance in several application areas, such as education, training, advertising and entertainment. Since multimedia documents may comprise continuous media, such as audio and video, the presentation of those documents may require a significant amount of processing and network resources. The amount of resources available during a presentation depends on the system configuration and the current system load. Hence, it can happen that there are not enough resources to render a multimedia document according to the specification, resulting in a reduced presentation quality, if the presentation is possible at all. To cope with those situations, different versions of the same document can be specified, one for each potential configuration or probable load situation. A better approach is to have only one document that can be adapted to different system configurations and load conditions. To enable this approach, an adaptive document model as well as an adaptive scheduling algorithm are necessary. In this paper, we present the adaptive Tiempo document model, an algorithm to check the consistency of specifications, the concepts of a graphical document editor supporting the model as well as a scheduling algorithm which allows to adapt documents conform to our model in environments with best-effort assignment of resources.