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Convert Color PDF Scan To Monochrome

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Table of Contents

Using ImageMagick convert utility
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Convert program throws an error, unless you change security policy by adding this line to /etc/ImageMagick-6/policy.xml

<policy domain="coder" rights="read | write" pattern="PDF" />

Threshold percentage
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Lower value allows more noise from non-white pixels, higher is more restrictive. Example:

convert -density 300 -threshold 70% 'rj my_input.pdf my_output.pdf

Alternative: Ghostscript
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I got it working for converting color to greyscale, but not for conversion to black and white. Example:

 gs  -sOutputFile=output.pdf  -sDEVICE=pdfwrite  -sColorConversionStrategy=Gray  -dProcessColorModel=/DeviceGray  -dCompatibilityLevel=1.4  -dNOPAUSE  -dBATCH  'input.pdf'

Alternative: pdfcpu
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Works with image files but not PDF or PS streams. Example:

pdfcpu import -- "gray:true" 'input.pdf' 'output.pdf'

Results in error:

input.pdf needs an image extension (.jpg, .jpeg, .png, .tif, .tiff, .webp)

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