Convert a PDF of JPGs to JPGs, recompress them, and then back to PDF.

pdf2jpg2pdf.sh

# Launch a fresh VM
docker run --rm -it -v `pwd`:/mount ubuntu

# Extract PDF to JPGs
cd /mount
apt-get update
apt-get install poppler-utils
pdfimages -j file.pdf fileprefix
mkdir extract
mv *.jpg extract

# Recompress JPGs and convert to greyscale
apt-get install imagemagick
FILES=$( find extract -type f -name "*.jpg" | cut -d/ -f 2)
mkdir shrink && cd shrink
for file in $FILES; do
    convert -strip -quality 30 -colorspace Gray ../extract/$file ./$file
done

# Back to PDF
# Note that your docker VM needs a lot of RAM to do this if you have a lot of images.
# Alternatively, if you're on a Mac, you can `brew install imagemagick` and
# do this last step native.
convert *.jpg ../final.pdf