During the construction of the European XFEL, detailed technical as-built documentation has been generated for the accelerator. It is stored in an Engineering Data Management System, which provides an inventory of major accelerator components and contains documents such as specifications, 3D-CAD models and drawings, work instructions, inspection and repair records, certificates and more. Inventory and documents are of great importance for maintaining the accelerator throughout its life-time. The paper provides an overview of the available engineering documentation, describes methods of using and updating the documentation, and discusses its expected role and benefits in future maintenance processes.