[thataccount]

Saturation curve in photo editing software possibly broken down by channels.

[gus_massa]

I agree. GIMP has a tool that shows the histogram of the number of pixels with each brightness and you can add apply a curve to make the background still almost white and the ink almost black. Some random tutorial from a google search https://www.gimp.org/tutorials/community/Basic_Color_Curves/ There are a few videos https://www.google.com/search?q=Saturation+curve+gimp

[@OP: I can give it a try on Monday, if I'm very bored and you are very lucky. Take a look in a few days, but I'm not a professional so expect a crappy result if any.]

Have you tried with ChatGPT? IIRC. My wife used it to delete the writing in a test of my daughter, so she could do it again to practice for school. I guess you can ask it to enhance the contrast.

Have you tried with the CamScanner app? It's like magic, but the adds are too aggressive and try to install other apps, so only 50% recomendation.

[gus_massa]

@OP: I tried but failed. In the main image the bottom left corner is darker. How did you took the photo? For slightly similar tasks I like natural light from a window in a cloudy day, so it's very diffuse. I take the photo far away from the paper to avoid shadows. It's better if you have a support for the phone/camera.

   O                     w
   |    ---camera        i
   |   /                 n
   me /                  d
   |        photo        o
   |   --------------    w
   |   |  table     |            
  / \  |            |