How to make a vector PDF searchable?

Question

My workflow includes making figures in Inkscape, which are then converted to PDF and included into LaTeX documents. In these figures, I often have to include mathematical formulas. For that, I use TexText. For font consistency and simplicity, when I want to add some plain text to my figure, I also use TexText. When the resulting SVG is converted to PDF, the TexText-generated text is not searchable.

How can I make a PDF from the SVG such that it is searchable while remaining a vector PDF?

I know I could rasterize the figure and then use e.g. Tesseract to create a searchable PDF. But the resulting PDF will of course contain a rasterized version of my figure. I would like the figure itself to remain vector graphics.

I am guessing there has to be a way that would go something like this: indeed rasterize the PDF and use Tesseract to extract the text. But then take the output of Tesseract and somehow add it to the original vector PDF. Unfortunately, I don't know how to do this.

Answer 1

It turns out a question that is directly relevant to mine was answered on another StackExchange, here. The script that answers my actual question is svgToSearchablePDF.sh, and I post it below. It uses, as the key element, the script pdf-merge-text.sh from the accepted answer to that other question. For completeness, I will repost pdf-merge-text.sh in this answer.

The solution

Note that perhaps you will need to magnify the SVG file before converting it to a searchable PDF: larger image sizes help the OCR process. To magnify, in Inkscape, select the entire image, then go to Object -> Transform… . In the Transform tab, select Scale. Then select 'Scale proportionally', and finally, in either 'Width' or 'Height', enter something like '300' (make sure % is selected in the menu immediately to the right of the 'Width' field). Next, File->Document Properties…. In the 'Document Properties' tab, under Custom size, click on Resize page to content. Make sure that either nothing is selected or else that the entire image is selected. Then click the button 'Resize page to drawing or selection'. Save the SVG.

The script svgToSearchablePDF.sh uses an SVG file as input and produces a searchable vector PDF file as output. It is assumed that all of the following are installed: Tesseract, Inkscape, and Ghostscript.

For example, assume that we used Inkscape to create the file mygraphics.svg. Then the following command will produce a searchable PDF file mygraphics.pdf:

svgToSearchablePDF.sh mygraphics.svg 

The scripts

First, svgToSearchablePDF.sh:

#!/bin/bash
filename="$1"

inkscape ${filename%.*}.svg -o ${filename%.*}_auxfile.png
inkscape ${filename%.*}.svg -o ${filename%.*}_auxfileVCT.pdf
tesseract ${filename%.*}_auxfile.png ${filename%.*}_auxfileTXT -l eng pdf
pdf-merge-text.sh ${filename%.*}_auxfileTXT.pdf ${filename%.*}_auxfileVCT.pdf ${filename%.*}.pdf


rm -f ${filename%.*}_auxfile.png ${filename%.*}_auxfileVCT.pdf ${filename%.*}_auxfileTXT.pdf

As I said, that script uses the script pdf-merge-text.sh from here. For completeness, here it is:

#!/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 "$@"