Another technique (better done in Gimp 2.10):
- Duplicate the layer, and set the top layer to Grain extract: the whole image turns a uniform gray.
- Filters>Blur>Gaussian blur and blur the top layer. The characters will re-appear in black. Increase the blur until the characters are dark and the halo around them is no longer visible but don't go too far otherwise the fold shadow will reappear.
- Layer>New from visible and use Levels or Curves to make the grey white, and the black very black