Scan and prepare the document
I scan the drawing in sections using a flat-bed A4 scanner. A setting of 600 DPI captures the details. In Photoshop I go to File>Automate>Photomerge and choose my scanned sections. This automatically merges all images and blend them together on separate layers. I create a new document (50x70cm and 300 DPI), drag the sections into it and merge them to one layer.