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?
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.
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
Adjust image quality. 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 or continue to use ImageMagick: Batch-processing images of documents to look like a fax
convert output*.jpg -normalize -threshold 80% final-%02d.jpg
If you want a pdf back:
convert final*.jpg my_new_highcontrast.pdf
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.
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
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
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.
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.
For Windows, I wrote a batch file
.bat to automatically generate the PDF upon drag-and-drop onto the batch file. (linux/OSX would require translating this to a shell script)
You first need to have ImageMagick (allow installer to add to PATH so
magick convert works from the command line) and GhostScript installed. Don't worry, they are easy to install.
Here is the batch file. Make a file
pdf_darkener.bat with the following contents:
@echo off echo Darkening the pdf at %1 echo Converting PDF to JPGs... magick convert -density 500 %1 darken_pdf_preprocess_%%02d.jpg echo Darkening JPGs... magick convert darken_pdf_preprocess*.jpg -level 50%%,100%%,0.3 darken_pdf_postprocess_%%02d.jpg echo Converting JPGs to PDF... magick convert darken_pdf_postprocess*.jpg darkened.pdf echo Deleting JPGs... del darken_pdf_*.jpg echo Done! pause
Drag any PDF file onto the batch file. Wait for operations to complete (slowness / level of detail is controlled by
density 500, higher = more pixels, bigger file size, slower). The output pdf will be in the same directory as the original pdf. The batch file cleans up after itself, deleting the intermediate jpgs created during processing.
Note that this contrast-increasing operation is controlled by three numbers:
50%%,100%%,0.3 which means 50%, 90%, and 0.3. These are black_level, white_level, and gamma, according to the ImageMagick documentation. black_level is the percentage of brightness below which everything gets mapped to pure black, visa-versa for white. Gamma controls a non-linear mapping for the in-between values (1.0 is linear).
Please share any improvements or useful modifications in the comments section.
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.