http://www.morethantechnical.com/2013/11/21/creating-a-searchable-pdf-with-opensource-tools-ghostscript-hocr2pdf-and-tesseract-ocr/

(second is better)make_pdf_searchable.sh

#!/bin/sh
 
# bash tut: http://linuxconfig.org/bash-scripting-tutorial
# Linux PDF,OCR: http://blog.konradvoelkel.de/2013/03/scan-to-pdfa/
 
y="$(pwd)/$1"
echo Will create a searchable PDF for $y
 
x="$(basename "$y")"
name=${x%.*}
mkdir "$name"
cd "$name"
 
# splitting to individual pages
gs -dSAFER -dBATCH -dNOPAUSE -sDEVICE=jpeg -r300 -dTextAlphaBits=4 -o out_%04d.jpg -f "$y"
 
# process each page
for f in $( ls *.jpg ); do
  # extract text
  tesseract -l eng -psm 3 $f ${f%.*} hocr
 
  # remove the “<?xml” line, it disturbed hocr2df
  grep -v "<?xml" ${f%.*}.hocr > ${f%.*}.noxml
  rm ${f%.*}.hocr 
 
  # create a searchable page
  hocr2pdf -i $f -s -o ${f%.*}.pdf < ${f%.*}.noxml
  rm ${f%.*}.noxml
  rm $f
done
 
# combine all pages back to a single file
# from http://www.ehow.com/how_6874571_merge-pdf-files-ghostscript.html
gs -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=../${name}_searchable.pdf *.pdf 
 
cd ..
rm -rf $name

make_pdf_searchable.sh

#!/bin/sh
 
# bash tut: http://linuxconfig.org/bash-scripting-tutorial
# Linux PDF,OCR: http://blog.konradvoelkel.de/2013/03/scan-to-pdfa/
 
y="$(pwd)/$1"
echo Will create a searchable PDF for $y
 
x="$(basename "$y")"
name=${x%.*}
mkdir "$name"
cd "$name"
 
# splitting to individual pages
gs -dSAFER -dBATCH -dNOPAUSE -sDEVICE=jpeg -r300 -dTextAlphaBits=4 -o out_%04d.jpg -f "$y"
 
# process each page
for f in $( ls *.jpg ); do
  # extract text
  tesseract -l eng -psm 3 $f ${f%.*} pdf

  rm $f
done
 
# combine all pages back to a single file
# from http://www.ehow.com/how_6874571_merge-pdf-files-ghostscript.html
gs -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=../${name}_searchable.pdf *.pdf 
 
cd ..
rm -rf $name