I’m unaware of any free tool to perform OCR on a PDF and embed the resulting data in the PDF itself so it is text-searchable. If anyone knows of one, let me know. In the meantime, I use Acrobat Professional for this essential functionality.
High resolution PDFs produced by my scanner (HP Officejet Pro L7700) usually give the following error when I try to perform Acrobat OCR:
This page is larger than the maximum page size of 45 inches by 45 inches.
Surprisingly, there doesn’t seem to be any way to resize the page size of a PDF within Acrobat. It’s possible to print to a new PDF of the correct size, but this operation cannot easily be batched. If I apply the “crop” tool to resize the page in Acrobat, I get this error:
Page size may not be reduced.
Many report these issues in Adobe’s forums. The most common responses suggest reconfiguring the scanner or buying a new one.
I found nothing quick and easy after some googling for a simple ghostscript recipe to perform the batch pre-processing necessary to allow Acrobat to do the OCR. It’s not hard to do, just a bit of a trial-and-error pain to get the right switches.
For posterity, then, here is a simple command-line to make this happen (here under Windows, but could obviously easily be adapted for any other platform). First, download the latest ghostscript for your platform (at this time, 8.64 for Windows). Then:
gswin32c -dQUIET -dNOPAUSE -dBATCH -sPAPERSIZE=letter -sDEVICE=pdfwrite -sOutputFile=OUTPUT.pdf -dPDFFitPage INPUT.pdf
And a simple inelegant script to batch process (again, under Windows/cygwin, but easily adaptable). Feel free to make more elegant:
#!/bin/bash for x in "$@" do echo -n Processing $x ... if [ ! -e "$x" ] then echo File $x missing. Exiting. exit 1 fi if [ -e gs_shrink_to_letter.pdf ] then echo Tempfile gs_shrink_to_letter.pdf exists. Exiting. exit 1 fi if ( gswin32c -dQUIET -dNOPAUSE -dBATCH -sPAPERSIZE=letter -sDEVICE=pdfwrite -sOutputFile=gs_shrink_to_letter.pdf -dPDFFitPage "$x" ) then echo Success. mv gs_shrink_to_letter.pdf "$x" else echo Error occurred, exiting. exit $? fi done
After converting your PDFs as above, you can then apply Acrobat batch OCR without a hitch.