[sorenjan]

Sounds like local contrast adjustment. There are several different ways of solving it, here's a couple that look like they work pretty well:

https://stackoverflow.com/questions/63251089/how-to-do-a-loc...

https://stackoverflow.com/questions/65666507/local-contrast-...

One possible preprocessing step could be to do a high pass filter on it, if the shadows vary slowly over the image.

There are also more specialized techniques specifically for removing shadows from documents, like these:

http://civc.ucsb.edu/graphics/Papers/ACCV2016_DocShadow/

https://faculty.iiit.ac.in/~vgandhi/papers/shadow_removal_ca...

I also found this, an image editor based approach if you just want to do a few images manually:

https://janithl.github.io/2021/12/remove-shadows-and-uneven-...

[Rygian]

Thanks! These pointers will be very helpful.