How to get the correct scale of pdf file?

Question:

When I attach pdf file into the model space and scale it up according to the drawing scale, for example, if the drawing is 1:100 at A3, I scale it by 100, why it is still not in the correct size?

Solution:

In this case, just simply scale it up by another factor 25.4, which is the magic factor between 'imperial' and 'metric'.