Paper to CAD is the procedure converting Paper Drawings (physical or scanned) into 3D CAD models and/or 2D Drawings.
Companies often have libraries of drawings or individual parts which were created prior to CAD (or 3D CAD) and only have the paper drawings on file.
The process of converting paper to CAD drawings is mainly used to make the new drawings editable. Often there is a set of drawings in PDF or Paper format which require updating. In order to do this, the existing drawings need to be reproduced in CAD.
These drawings can either be converted like for like in 2D CAD such as AutoCAD or by producing a 3D CAD Model first and then extracting a 2D Drawing.