In this paper, we propose a process model, which we call Document Product Lines, for the intensive generation of documents with variable content. Unlike current approaches, we identify the variability sources at the requirements level, including an explicit representation and management of these sources. The process model provides a methodological guidance to the (semi)automated generation of customized editors following the principles, techniques, and available technologies of Software Product Line Engineering. We illustrate our proposal with its application to the intensive generation of Emergency Plans.