Multimedia documents are collections of media objects, synchronized by means of sets of temporal and spatial constraints. Any multimedia document definition is valid as long as the referred media objects are available and the constraints are satisfiable. Document validity depends on the context in which the document has to be presented. In this paper, we introduce a framework to characterize context adaptation, in the presence of both physical and user oriented context requirements. We define semantically equivalent presentation fragments as alternative to undeliverable ones. In the absence of equivalence, undeliverable media are replaced with candidates that minimize the loss of information/quality in the presentation.