Flatten PDFs with ghostscript (GS를 사용해서 PDF를 flattening할 수 있습니다. MacOS Sierra에서 테스트하였습니다.)

flatgs.sh

# http://zeroset.mnim.org/2015/01/07/flatten-pdfs-with-ghostscript/
# https://unix.stackexchange.com/questions/162922/is-there-a-way-to-flatten-a-pdf-image-from-the-command-line

# There should be `flat` sub-directory.

for file in *.pdf; do
	gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite \
	-sColorConversionStrategy=/LeaveColorUnchanged  \
	-dAutoFilterColorImages=true \
	-dAutoFilterGrayImages=true \
	-dDownsampleMonoImages=true \
	-dDownsampleGrayImages=true \
	-dDownsampleColorImages=true \
	-sOutputFile="flat/flat-$file" "$file" 
done