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I've tried opening the Accessibility menu and choosing black background with green font, but it seems this does not work if the document was originally scanned in Black/White mode. Is there any other way to enhance the PDF's contrast?

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    Unless you ran OCR on the scan, there is no text, just a picture of text, so no text-related settings will affect anything. – fixer1234 Jul 7 '16 at 21:46
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    Extract images (scanned pages) from PDF, use some image manipulation to enhance contrast, put images back. I don't know of any PDF viewers that allow you to manipulate image contrast directly. – dirkt Sep 13 '16 at 9:49
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If it has tons of pages, the easier tool is a command line one: http://www.imagemagick.org/script/download.php

(ImageMagick is a very popular image manipulation library.)

You will have to do three steps.

  1. Convert PDF pages to individual image files. See: Convert PDF to image with high resolution or Convert PDF to JPG images with ImageMagick - how to 0-pad file names?

    convert -density 600 your_pdf_filename.pdf  output-%02d.jpg
    
  2. Adjust image quality. You can do that manually with Photoshop or GIMP, or continue to use ImageMagick: Batch-processing images of documents to look like a fax

    convert output*.jpg -normalize -threshold 80% final.jpg
    
  3. If you want a pdf back:

    convert final*.jpg very_readable.pdf
    

If you have only a few pages, Photoshop or GIMP (free and open-source alternative to Photoshop) will simply import each page as an image. Update the contrast as you'd like and save. For more info see Gimp: how to remove background noise/artifacts and enhance handwritten text

  • Problem with this is, if you have lots of bookmarks in the pdf, they are all lost. – bgmCoder Apr 5 at 2:12
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I have a PDF with a lot of gray images (manga). So I used the answer of gcb, changing the 2nd command to:

convert output.jpg -level 25% output_contrast.jpg

Just change the level value percentage to what serves you.


Observation.

With the arg -threshold you get a "black and white" (only) image. But I want to keep the gray scale, which is possible with the arg -level: you keep the gray, letting the image with a darker or lighter gray scale.


The commands order will be:

convert your_pdf_filename.pdf output-%02d.jpg
convert output*.jpg -level 25% final-%02d.jpg
convert final*.jpg very_readable.pdf
  • So what is the advantage of this variation compared the answer by gcb? – Blackwood Dec 14 '17 at 13:48
  • With "threshold" you get a "black and white" (only) image. With "level" you keep the gray, letting it "darker" or "lighter" – Zack Dec 14 '17 at 14:37
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    That would be good information to edit into your answer. – Blackwood Dec 14 '17 at 14:41
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Nuance Paperport has the ability to increase contrast on scanned pdf pages.

The free software NAPS2 is also able to do this, either in combination with scanning or after importing a scanned page. NAPS2 also has other features, e. g. to crop and to automatically deskew a scanned page. On my machine (Core i5, 8 GB) import is a bit slow, but the page editing seems to work fine.

  • NAPS2 works great - it is easy enough and intuitive, but if you have any bookmarks already in the pdf, they will all be lost. – bgmCoder Apr 5 at 2:20
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You can convert pdf to jpg online . Adjust pictures on a photo editor. I used Adobe Lightroom to adjust contrast on multiple photos at once with synchronized settings to save time. Then convert JPG to PDF online

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If you are not comfortable with Image Magick and command line and want more control over the process, I suggest using Foxit PdfPhantom (Business Version) along with adobe Photoshop. You can simply convert your pdf to any image format that you want with Foxit under the Convert tab. Then you can use Photoshop to apply a specific sequence of operations to a series of photos. For this purpose, see this tutorial. Finally, you can turn back to pdf using Foxit.

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I do not think that any of the PDF viewers have built in filters or adjustments for pictures, only for fonts backgrounds and colors.

You could adjust your monitor, and/or if your video card software supports it adjust the video cards, video settings. Save the video settings in a profile to use again, for this purpose, it would not be too bad.

You could print it and adjust your printer (most printer preferances allow adjusting).
You could Screen grab it , then paste it into a picture/photo editor and adjust.
If your "allowed" and have PDF creation program you could fix each picture used, and remake it.
If you have Adobees photoshop program, some versions of it would open a PDF, I do not know what it could do.

See also Adjust PDF Contrast with Adobe Reader/Acrobat?
It has some very good and easy answers if the PDF is not locked down.

You did not indicate if it is one picture, or many, which would help the question. You did not indicate the operating system (even a tag for it works). adding that information to the question will improve being able to answer.

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