Multimedia documents are of importance in several application areas, such as education, 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. To cope with those situations, documents can integrate presentation alternatives permitting to adapt the document presentation to different system configurations and load conditions. In this paper, we present an adaptive scheduling algorithm which allows to adapt documents conform to our document model Tiempo in environments with best-effort assignment of resources.