In addition to some other points, the downscaling step in #1 may also smooth out some noise in the source image. Less noise yields more compressible data.
In my own scanning workflow, I scan at 600 or 1200dpi to png, deskew, downscale, and apply a black/white threshold. This is all done with imagemagick: mogrify -deskew 40% -scale 25% -threshold 50%
In my own scanning workflow, I scan at 600 or 1200dpi to png, deskew, downscale, and apply a black/white threshold. This is all done with imagemagick: mogrify -deskew 40% -scale 25% -threshold 50%
If I want a PDF after that I'll use img2pdf.