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We get PDF's from our professor to read for homework but they're often scanned documents, is there a way to adjust the contrast of the text to make it easier to read?

Edit: I've got Photoshop but is there a way to do it from a PDF reader?

Edit2: Windows XP, 7 ** Windows or Ubuntu Only **

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I'd suggest asking your professor to scan it as black & white instead of grayscale or 24-bit color, as long as the content is mostly text and line art (i.e., no photos or gradients). Not only will this make the documents perfectly legible, but it will also shrink the file size dramatically. Maybe you can turn this to your advantage and get some extra credit in return for rescanning or fixing the contrast on all the PDFs. –  rob Mar 8 '10 at 19:04
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10 Answers

You can try this:

Go to Edit>Preferences>Accessibility

This will not change the true contrast, but you can pick contrasting colors of your choice, or one of the defaults, as in the screenshot.

enter image description here

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I have tried this but it doesn't work. I think this works only for ebooks. My pdf is scanned text, so it's made of images. When do accessibility thing, the whole page goes white. It doesn't recognize the text. –  becko Aug 5 '11 at 5:09
    
I see. If it is a scanned image, I do not think there will be much you can do. I would mention it to the professor so he can consider adding contrast before he makes it a PDF. –  KCotreau Aug 5 '11 at 5:15
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Worked great for me though, so still a +1 –  Ivo Flipse Aug 19 '11 at 20:07
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You could try Imagemagick - it's a graphics manipulation program that can read and write PDFs too.

There are a few command line options that may help - for example: -normalize, -contrast and -contrast-stretch

http://www.imagemagick.org/scriptindex.php

Try something like: convert original.pdf -contrast new.pdf

More info and examples on the site.

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Good answer, and one that works fine; but note that ImageMagick doesn't do the PDF interpretation on its own -- in the background it employs Ghostscript as its delegate doing some of the hard work... –  Kurt Pfeifle Aug 8 '11 at 19:38
    
could not get this trick to work -- looks like ghostscript errored out (screenful of mostly incomprehensible text, but near the end: "postscript delegate failed 'thefilename.pdf' No such file or directory (but it's there) –  JMarsch Mar 27 at 14:11
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What I did was change the color of the text by going to edit menu, clicking on preferences and then the accessibility tab. You can customize the color for the document text. It doesn't do anything for the images, but at least you can see the text on a dark background.

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I saved it as a Microsoft Word file in Acrobat Reader. Then I opened the Word Document and adjusted the brightness and contrast of the image until it was readable. It makes for an expensive print, but it works.

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Under OS X, you can use ColorSync which is installed by default. There are many filters, and one is for decreasing contrast.

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If you open it in Photoshop and resave as a PSD file you can or if you want to apply on all pages, do the following:

  1. Open the pdf in Preview.
  2. Make sure the preview pages are showing
  3. Click page one
  4. "Select All" (or Command + A), All pages are selected
  5. Run any Preview tool/filter and all pages will be affected simultaneously

If your pdf is locked, you will not be able to perform this operation.

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I don't want to sound negative, but he does state he would like to do it with a PDF reader ;-) Photoshop is a quite expensive PDF reader! –  Ivo Flipse Mar 8 '10 at 18:02
    
I know... it is :) but the feature requested is not in new version unfortunately. Hope my answer will help him since he has Photoshop. –  r0ca Mar 8 '10 at 18:27
    
I used to use Photoshop to try to fill out forms before I learned of other options, regardless, my experience with photoshop and pdf makes me fear ever doing that again. Thank tho. –  wag2639 May 14 '10 at 21:50
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I've run into this as well. for some reason version 5 of the reader seems to work as expected. I think it might be something in the creation, but I've not tracked it down yet.

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I changed contrast with PDFClerk. It has a lot of filters in there, when exporting PDF.

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Thanks but I should have mentioned this is for a Windows machine. –  wag2639 May 14 '10 at 21:47
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You can use the graphics card or monitor settings to handle this.

See this post as well.

For that you could use a system wide gamma/brightness/contrast setting; usually if you have a modestly advanced graphics card, its control panel will have options to change gamma / contrast / brightness / hue. e.g. NVIDIA control panel, ATI Catalyst Control Center/Panel etc. It will affect the the whole system, but you can always change it back when you're done viewing the file.

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You can also try Nuance Paperport (I got this "free") with my Brother MFC scanner/printer.

Essentially if you have a dark grey font on a light grey background (a low contrast scan), then you tell it to "stretch" the dark grey to black and the light grey to white. This is done by setting the black/white points as follows:

  • Open Paper port, navigate to the PDF
  • Right click the PDF within PaperPort
  • "SET Tools"
  • Auto-enhance (or Apply current white/black points)

Now you can manually set the while/black points for finer control

  • Open Paper port, navigate to the PDF
  • Right click the PDF within PaperPort
  • Open with ImageView
  • In the top ribbon, pick "White point" and now click some area of the scanned page you think should be white (eg: grey background of a low contrast scan).
  • In the top ribbon, pick "Black point" and now click some area of the scanned page you think should be black (eg: The grey text letters of a low contrast scan)

I used this to create a legible black and white PDF from a scan that was originally black text on dark blue paper (that scans as a very low contrast)

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protected by Diago Nov 8 '10 at 11:19

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