Reduce text color to two colors

Question

I've scanned a book containing only black and red text, no other colors. Then I've modified it with Adobe Acrobat Pro DC, that has recognized the text. However, when I look at the the very color palette, I see that actually there are different kinds of black and different kinds of red. My typographer asked me to reduce all of them to only one red and one black. If I do it manually - and for a lot of text - by "Edit PDF" tool, it often changes the layout together with the color.

I'm not very skilled in color management. How to do it?

simonec33551593 · Accepted Answer

Well, I have very bad news for you: Redo the book. I had a similar issue on a book, still done on a typewriter. Someone retyped the text, I redid the typesetting.

I've found a solution that works for me:

Import every single page of the PDF into GIMP, with high resolution (1000px at least).
Create a custom palette that contains only the colors you want (in my case Red, White and Black)
Apply the indexed mode color conversion using the custom palette: https://docs.gimp.org/en/gimp-image-convert-indexed.html
File -> Overwrite file.pdf
Re-open it with Adobe Acrobat and, since it's still in high resolution, it will recognize the text again, but with the good colors.

Before:

Indexed color conversion:

After:

The loss of detail may depend on the resolution of the imported PDF and on the dithering settings.

Hope it helps.

SC

Test Screen Name · Answer

Ooh, that sounds hard. But my question - maybe I misunderstand - is, if you want to produce a copy of a scanned book, why did you recognise text?

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