Thomas Weise (汤卫思)

A professor at Hefei University.

3 minute read

Sometimes we have PDF document and want to submit through a web form, but it is too big. Often, this size is caused by the images inside the PDF. We may reduce their resolution, their “dots per inch” (DPI) value, and then the PDF gets smaller. This can be done via Ghostscript. Here I provide the little script scalePdf.sh, which does this in the terminal. It basically just executes Ghostscript with the correct parameters.

All you have to do is to provide the path to the source PDF document and the DPI to which you want to scale it. If you call scalePdf.sh mydoc.pdf 200, then it will create a file mydoc_200dpi.pdf, which is the same document, but scaled to 200 DPI. Additionally, you can also provide a destination path as optional third argument. Invoking scalePdf.sh mydoc.pdf 33 ../result.pdf creates the document result.pdf in the parent folder of the current folder. This new document would be the same as mydoc.pdf, but with all images inside scaled to 33 DPI. You can play around with different resolutions (DPI values) until you reach an acceptable file size.

If Ghostscript is not yet installed, the script will automatically install it. Here you can download this script and the complete collection of my personal scripts is available here.

#!/bin/bash -

# (Down)scale a PDF file to the given DPI value.
#
# The script expects the following parameters:
# 1. The path to a source document.
# 2. The DPI value to scale to.
# 3. Optional: The output file name.
#
# If the output file name is not specified, we will create a document in the
# current directory based on the input document name, i.e., "123.pdf" becomes
# "123_200dpi.pdf" if 200 were specified as second parameter.

# strict error handling
set -o pipefail  # trace ERR through pipes
set -o errtrace  # trace ERR through 'time command' and other functions
set -o nounset   # set -u : exit the script if you try to use an uninitialized variable
set -o errexit   # set -e : exit the script if any statement returns a non-true return value

srcDocument="$(realpath "$1")"
dpi="$2"
dstDocument="${3:-}"
if [[ -n "$dstDocument" ]]; then
  dstDocument="$(realpath $dstDocument)"
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): Scaling '$srcDocument' to the specified destination document '$dstDocument' using $dpi dpi."
else
  dstDocument="$(basename "${srcDocument%.*}_${dpi}dpi.pdf")"
  dstDocument="$(realpath $dstDocument)"
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): No destination document specified, therefore converting '$srcDocument' to  '$dstDocument' using $dpi dpi."
fi

package="ghostscript"
if dpkg-query -W -f='${Status}' "$package" 2>/dev/null | grep -q "ok installed"; then
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): '$package' is installed, so we can use it."
else
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): '$package' is not installed. We install it now. This needs to be done only once."
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): We first update the package cache. This requires sudo privileges."
  sudo apt-get update -y
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): We now install '$package'. This requires sudo privileges."
  sudo apt-get install -y "$package"
  echo "$(date +'%0Y-%0m-%0d %0R:%0S'): The installation of '$package' is finished."
fi

echo "$(date +'%0Y-%0m-%0d %0R:%0S'): Now piping document '$srcDocument' through '$package', creating '$dstDocument'."

gs -dAntiAliasColorImages=true \
   -dAntiAliasGrayImages=true \
   -dAntiAliasMonoImages=true \
   -dAutoFilterColorImages=false \
   -dAutoFilterGrayImages=false \
   -dAutoRotatePages=/None \
   -dBATCH \
   -dCannotEmbedFontPolicy=/Error \
   -dColorConversionStrategy=/LeaveColorUnchanged \
   -dColorImageDownsampleType=/Bicubic \
   -dColorImageFilter=/FlateEncode \
   -dColorImageResolution="$dpi" \
   -dCompatibilityLevel="1.7" \
   -dCompressFonts=true \
   -dCompressStreams=true \
   -dCreateJobTicket=false \
   -dDetectDuplicateImages=true \
   -dDoThumbnails=false \
   -dDownsampleColorImages=true \
   -dDownsampleGrayImages=true \
   -dDownsampleMonoImages=true \
   -dEmbedAllFonts=true \
   -dFastWebView=false \
   -dGrayImageFilter=/FlateEncode \
   -dGrayImageDownsampleType=/Bicubic \
   -dGrayImageResolution="$dpi" \
   -dHaveTransparency=true \
   -dMaxBitmap=2147483647 \
   -dMonoImageDownsampleType=/Subsample \
   -dMonoImageResolution="$dpi" \
   -dNOPAUSE \
   -dOptimize=true \
   -dPassThroughJPEGImages=true \
   -dPassThroughJPXImages=true \
   -dPDFSTOPONERROR=true \
   -dPDFSTOPONWARNING=true \
   -dPreserveCopyPage=false \
   -dPreserveEPSInfo=false \
   -dPreserveHalftoneInfo=false \
   -dPreserveOPIComments=false \
   -dPreserveOverprintSettings=false \
   -dPreserveSeparation=false \
   -dPreserveDeviceN=false \
   -dPreserveMarkedContent=false \
   -dPrinted=false \
   -dOmitInfoDate=true \
   -dOmitID=true \
   -dOmitXMP=true \
   -dQUIET \
   -dSAFER \
   -dSubsetFonts=true \
   -dUCRandBGInfo=/Remove \
   -dUNROLLFORMS \
   -sDEVICE=pdfwrite \
   -sOutputFile="$dstDocument" \
   "$srcDocument" \
   -q

echo "$(date +'%0Y-%0m-%0d %0R:%0S'): Finished converting '$srcDocument' to '$dstDocument'."

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