Don't forget that Product Lifecycle Management (PLM, of which PDM is a primary area) is a discipline by itself, even covered by a couple of big standards (e.g. the famous ISO 10303 series for product data representation and exchange). PLM usually has it's own information systems (not identical with the ERP systems) which manage all product-related data including CAD/CAM files. PLM systems are used in the product development process whereas ERP systems are used in the production and order delivery process.