Is there a way to increase the contrast of a PDF that was created by scanning a photocopy in B-W

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.

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. 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
   

3. 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.

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

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.