Pdf – When running Adobe Acrobat’s OCR on a PDF document, which downsampling produces a higher quality: 600 dpi or 72 dpi

ocrpdf

I have a large PDF document that consists of scanned pages of a textbook. I want to run Adobe Acrobat 9's text recognition function on it, but I'm presented with this menu when I do.

Adobe Acrobat OCR Menu

I'm confused by the options in the highlighted menu. What option will produce the highest quality/most readable text? I thought 600 dpi implies a higher quality image than 72 dpi, so I'm confused by "High (72 dpi)" and "Lowest (600 dpi)."

Best Answer

The qualitative description of your options is referring to the amount of downsampling done to the image, not the resulting image quality. Downsampling to 72 dpi results in a high amount of downsampling.

Here's a simple shell script to do this on the command-line:

Save this as ~/pdf-merge-text.sh (and chmod +x it):

#!/usr/bin/env bash

set -eu

pdf_merge_text() {
    local txtpdf; txtpdf="$1"
    local imgpdf; imgpdf="$2"
    local outpdf; outpdf="${3--}"
    if [ "-" != "${txtpdf}" ] && [ ! -f "${txtpdf}" ]; then echo "error: text PDF does not exist: ${txtpdf}" 1>&2; return 1; fi
    if [ "-" != "${imgpdf}" ] && [ ! -f "${imgpdf}" ]; then echo "error: image PDF does not exist: ${imgpdf}" 1>&2; return 1; fi
    if [ "-" != "${outpdf}" ] && [ -e "${outpdf}" ]; then echo "error: not overwriting existing output file: ${outpdf}" 1>&2; return 1; fi
    (
        local txtonlypdf; txtonlypdf="$(TMPDIR=. mktemp --suffix=.pdf)"
        trap "rm -f -- '${txtonlypdf//'/'\\''}'" EXIT
        gs -o "${txtonlypdf}" -sDEVICE=pdfwrite -dFILTERIMAGE "${txtpdf}"
        pdftk "${txtonlypdf}" multistamp "${imgpdf}" output "${outpdf}"
    )
}

pdf_merge_text "$@"

Now just call it:

~/pdf-merge-text.sh txt.pdf img.pdf out.pdf

The idea is to strip images from the OCR'd PDF, then merge it via the the technique in the answer above.

Best Answer

Related Solutions

Better way to correct errors in Adobe Acrobat’s OCR results

Copy pdf text layer to another pdf

Here's a simple shell script to do this on the command-line:

Related Question