As this is a photography space, let's talk about light and shadow.
Use the flattest - flattest - flattest light you can.
If you have a difference of light, for example, due to falling off, you will have a really hard time trying to add contrast or adjust the curves or levels.
If you have a point light, the texture of the paper could also make small shadows, so use different light from different angles.
Flatten the paper, if you can with a glass. Again, every protuberance of the paper will produce a difference in light, making it hard to make adjustments in post.
Probably the best option is to use a flatbed scanner; it will produce the flattest light. But still, depending on the paper texture it could produce "micro shadows" because the light is directional.
There is a chance that you could use a different type of light, for example, UV light, but that is beyond my expertise.