Epson scanner, large PDFs and ImageMagick (convert CLI)

The Epson iPrint soft is producing PDFs in 300dpi but the 300 information does not seem to be stored in the PDF metadata; thus, the PDF reader thinks by default that it is a 72 dpi PDF and thus the huge size:

87,49 × 123,72 cm

instead of an A4 paper:

21 × 29,7 cm

To convert this 300 dpi into a real 300 dpi:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -dDetectDuplicateImages -dCompressFonts=true -sPAPERSIZE=a4 -dPDFFitPage -sOutputFile=output.pdf input.pdf

• /screen selects low-resolution output similar to the Acrobat Distiller (up to version X) “Screen Optimized” setting.
• /ebook selects medium-resolution output similar to the Acrobat Distiller (up to version X) “eBook” setting.
• /printer selects output similar to the Acrobat Distiller “Print Optimized” (up to version X) setting.
• /prepress selects output similar to Acrobat Distiller “Prepress Optimized” (up to version X) setting.
• /default selects output intended to be useful across a wide variety of uses, possibly at the expense of a larger output file.

See:

open /usr/local/share/ghostscript/9.22/doc/index.html

(WORKS BUT BIGGER SIZE AFTERWARDS)
convert -units PixelsPerInch -density 300 filein.pdf fileout.pdf
convert -units PixelsPerInch EPSON004.pdf -density 300 -quality 30 -compress jpeg fileout.pdf

Color to grayscale

From: https://superuser.com/questions/104656/convert-a-pdf-to-greyscale-on-the-command-line-in-floss

while read i; do
    gs -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH -sOutputFile="${i/.pdf/_gray.pdf}" "$i";
done <<< "some.pdf"

Using ImageMagick (mogrify):

mogrify -despeckle -fuzz 5% -fill white -opaque white -gamma 0.8 -colorspace gray -depth 6 -format png \*.jpg

OCR

To black and white

A SO link talking about that: https://stackoverflow.com/questions/15211428/conversion-of-tiff-to-pdf-with-ghostscript.

gs -dQUIET -dNOPAUSE -r200 -dBATCH -sPAPERSIZE=a4 -sDEVICE=tiffg3 -sOutputFile=temp.tiff -sColorConversionStrategy=Mono -sColorConversionStrategyForImages=/Mono -dProcessColorModel=/DeviceGray "a.pdf"
tiff2pdf -o out.pdf -p A4 -F temp.tiff

Or:

pdfimages -j "some.pdf" out/
cd out/
mogrify -despeckle -fuzz 5% -fill white -opaque white -gamma 0.8 -colorspace gray -depth 6 -format png \*.jpg

Remove noise created by scanning text in JPEG

pdfimages -j "some.pdf" out/
cd out/
mogrify -despeckle -fuzz 5% -fill white -opaque white -gamma 0.8 -colorspace gray -depth 6 -format png *.jpg