How to remove background text from an image file
It is all about filtering, you can see there are 4 colors, i.e. C_Font, C_Paper, C_Watermark_Paper, and C_Watermark_Font, the filter is just f(C_Watermark_Paper) = C_Paper and f(C_Watermark_Font) = C_Font.
Of course you can use GIMP and easily Google references, i.e. http://howto.nicubunu.ro/gimp-remove-watermark/