When a drawing is scanned it does not become CAD. It is a ‘raster’ image – just a series of dots of different colors
and shades. It cannot be edited in CAD because CAD programs use vectors, not dots, for their lines.
The illustration below shows the difference between vectors and rasters when you look up close. If you open a vector image and zoom in on a detail, the lines appear clearly, no matter how far in you zoom. If you
open a raster image and zoom in on a detail the lines appear jaggy.
The raster image appears jaggy because the lines have been created using individual 'dots'. When the dots are seen together on a screen our brains 'connect the dots' and we see lines. Raster images are created when a drawing is scanned into a computer. Many PDFs are raster images because they were created by scanning in the first place. A photo is another example of a raster image.
A raster is usually fine for viewing images, provided the original raster image is of a sufficiently high resolution. By this we mean that there are plenty of dots per inch (dpi). Our brains will do the connecting work.
CAD programs work completely differently. They describe lines mathematically. A CAD program defines a straight line as "start at coordinate x1,y1 and go to coordinate x2,y2". Curved lines and other shapes are defined using more complex mathematical descriptions. You will often see the statement that CAD drawings are "intelligent" because they contain precise dimensions and entity types. This is a misuse of the word since only living creatures have intelligence - but you can see why it is used. A more accurate description of the content of a CAD file is "rich data".
When the time comes to edit a scanned plan or engineering drawing there are four possible approaches: