How to convert PDF to Grayscale

Question

I scanned a document in color to PDF, and I don't want to re-scan it.

How can I convert this PDF document to grayscale (preferably without loss of other information)?

My usual toolset of doPDF, pdfsam and the Neevia online converter don't work here. I was thinking I could simply print to PDF and select the Grayscale color option, but doPDF doesn't have that option.

I'm using Windows, but I can probably handle a solution for any OS.

Answer 1

I found a solution in a blog post:

With GhostScript (and Windows users can access gs via cygwin), you can do it with the following command:

gs \
  -sOutputFile=grayscale.pdf \
  -sDEVICE=pdfwrite \
  -sColorConversionStrategy=Gray \
  -dProcessColorModel=/DeviceGray \
  -dCompatibilityLevel=1.4 \
   source.pdf

This will convert source.pdf to grayscale.pdf and put you into a GhostScript shell. To exit, press return and then enter quit in the shell.

Page 1
>>showpage, press <return> to continue<<

GS>quit

Alternately, you can pipe null in as input and that will convert the PDF and not leave you in the GhostScript shell.

command < /dev/null

Or, in a Windows command prompt:

command < NUL

Edit: from a comment, an improvement (for this case) in the arguments you can pass:

gs \
  -o grayscale.pdf \
  -sDEVICE=pdfwrite \
  -sColorConversionStrategy=Gray \
  -dProcessColorModel=/DeviceGray \
  -dCompatibilityLevel=1.4 \
   source.pdf

(This will circumvent the need for dealing with the GS prompt.)

Answer 2

Sounds like a job for Imagemagick. Windows and Linux versions are available.

http://www.imagemagick.org/script/index.php

This should do the trick at the command line:

convert -colorspace GRAY original.pdf grayscale.pdf

Answer 3

Just install PDFCreator. It has a small footprint and adds a PDF printer to Windows. And as you've mentioned, just select grayscale when printing to it.

Answer 4

You can just use the free service at http://greyscalepdf.com to convert any PDF to greyscale.