Lighten those pages

• convert output.pdf -function polynomial 1,0,0,0 darkened.pdf
• mogrify output.pdf -contrast-stretch 2%x20% music1C.pdf
• convert -density 600 output.pdf output-%02d.jpg

(using rm to add security permissions then remove them after https://stackoverflow.com/questions/52861946/imagemagick-not-authorized-to-convert-pdf-to-an-image)

sudo mv /etc/ImageMagick-6/policy.xml /etc/ImageMagick-6/policy.xml.off

-----When done, you can restore the original with

sudo mv /etc/ImageMagick-6/policy.xml.off /etc/ImageMagick-6/policy.xml

3 step process:

• convert your_pdf_filename.pdf output-%02d.jpg
• convert output*.jpg -level 25% final-%02d.jpg
• convert final*.jpg very_readable.pdf

(change the level value)

With the arg -threshold you get a "black and white" (only) image. But I want to keep the gray scale, which is possible with the arg -level: you keep the gray, letting the image with a darker or lighter gray scale. (referring to something like <<< convert output*.jpg -normalize -threshold 80% final-%02d.jpg >>>