Changes DPI of a scanned PDF document using ghostscript

change-dpi.sh

#!/bin/bash

# google gemini wrote this
# uses gostscript (sudo apt install ghostscript)

# 1. Check if the user provided all 3 arguments
if [ "$#" -ne 3 ]; then
    echo "Usage: $0 <input_file> <output_file> <dpi>"
    echo "Example: $0 input.pdf compressed.pdf 300"
    exit 1
fi

INPUT_FILE="$1"
OUTPUT_FILE="$2"
TARGET_DPI="$3"

# 2. Check if the input file actually exists
if [ ! -f "$INPUT_FILE" ]; then
    echo "Error: Input file '$INPUT_FILE' not found."
    exit 1
fi

echo "Compressing '$INPUT_FILE' to ${TARGET_DPI} DPI..."

# 3. Run the Ghostscript command
gs \
 -sDEVICE=pdfwrite \
 -dCompatibilityLevel=1.4 \
 -dNOPAUSE \
 -dQUIET \
 -dBATCH \
 -dDownsampleColorImages=true \
 -dColorImageResolution="$TARGET_DPI" \
 -dDownsampleGrayImages=true \
 -dGrayImageResolution="$TARGET_DPI" \
 -dDownsampleMonoImages=true \
 -dMonoImageResolution="$TARGET_DPI" \
 -sOutputFile="$OUTPUT_FILE" \
 "$INPUT_FILE"

# 4. Confirm success
if [ $? -eq 0 ]; then
    OLDSIZE=$(wc -c "$INPUT_FILE" | cut -d " " -f 1)
    OLDSIZEHUMAN=$(du -h "$INPUT_FILE" | cut -f 1)
    NEWSIZE=$(wc -c "$OUTPUT_FILE" | cut -d " " -f 1)
    NEWSIZEHUMAN=$(du -h "$OUTPUT_FILE" | cut -f 1)
    RATIO=$(awk "BEGIN {printf \"%.2f\", 100 * $NEWSIZE / ($OLDSIZE + 0.001)}")
    echo "Success! Saved to '$OUTPUT_FILE'."
    echo "Old size: $OLDSIZEHUMAN"
    echo "New size: $NEWSIZEHUMAN ($RATIO %)"

else
    echo "Error: Something went wrong during compression."
    exit 1
fi