How can I simultaneously darken all black text in a PDF of an old scanned book?

Question

I prefer a free solution that uses just Adobe Acrobat or Reader. If other software is necessary, I have GIMP. I don't have Adobe Photoshop. It's doubtless too unproductive for me to edit each page; the solution must automatically concurrently blacken all text.

RA Duff's Intention, Agency and Criminal Liability can be downloaded freely and safely from SSRN. It was "originally published in 1990, now out of print". I screenshot one page. As you can see below, the text is in light gray, but I fancy pure black.

I read these Mar 8 2010 and Jul 23 2013 Super User questions, Chron.com updated June 13 2019, Acrobat Library, but I wonder if they're outmoded.

Answer 1

I looked into this because I sometimes have the same need and I found a solution using only a Fixup in Acrobat which can apply a curve to each image of the PDF.

1. Open any PDF file in Acrobat.

2. Open the Preflight tool.

3. Click the Select single fixups button.

4. In the Options drop-down choose Create Fixup....

5. Name the new fixup something like "Darken Scanned Text".

6. Under Type of fixup choose Adjust dot gain.

7. Click the Dot gain curve setting drop-down and select Open folder with configuration files.

8. The folder with the curve files open. Make a duplicate of one of the files, rename it to "Darken Scanned Text.crv" and open it in a text editor.

9. Edit the file as follows to create a curve which darkens the images so everything above 30% black becomes 100% black (you can copy/paste from below. Make sure to preserve the tab characters as they are):

   DisplayName	1	Darken Scanned Text
   INPUT	DEFAULT
   0.0	0.0
   0.1	0.0
   0.3	1.0
   1.0	1.0

10. Save the file and return to Acrobat.

11. Be sure you have selected the right options, by using the check marks, amongst (1) Apply to device dependent CMYK and spot colors, (2) Apply to device dependent RGB, (3) Apply to device independent colors.

12. Sadly you can't choose the newly created .crv file before restarting Acrobat, so just choose some other curve for now and click OK to save the fixup.

13. Close Acrobat and open the PDF file you want to edit in Acrobat.

14. Open the Preflight tool again.

15. Find the "Darken Scanned Text" fixup we created before and click its Edit button.

16. Now in the Dot gain curve setting drop-down our "Darken Scanned Text" curve should appear. Choose it and click OK to save the fixup.

17. Make sure the "Darken Scanned Text" fixup is selected and click Fix.

I'm getting the following result. If you are not satisfied with the result you can try tweaking the curves file.

Answer 2

I have to print badly scanned PDFs several times a week, and I was getting tired of wasting toner in my printer because of all of the black page edges.

Here's the approach I ended up taking. It's a little bit more involved, but I'm overall very happy with the results.

1. Extract all of the PDF pages as PNGs. I use pdftoppm for this.
2. Use ScanTailor to crop, straighten, standardize page sizes, and clean up the visual appearance of the pages.
3. ScanTailor outputs tif files. To combine these into PDFs, I use tiffcp and tiff2pdf from the libtiff library.
4. (Optional) I use pdfnup to create a PDF with multiple pages per page, which can be convenient when printing the resulting file.

I use Ubuntu, and I've created scripts for steps 1, 3, and 4. (They also use R, since it's what I'm most comfortable with, but you can easily convert it to bash.) The only step that requires manual review is the ScanTailor step, but ScanTailor itself is pretty fast. Re-processing a PDF like the one you shared just takes a couple of minutes (it took me longer to write this response, actually), and the results are like the following:

Here's a sample of the output with 2 pages per page:

The resulting PDF file was about 8.6 MB (using 300 dpi for the output from ScanTailor).

Answer 3

Some workarounds only. I haven't modern Acrobat Pro, so this cannot be considered full answer.

The PDF contains spread wide JPG images. You can extract image files from the PDF with some PDF exploding program. I tried it with PDFExtractor. It produced into one folder the spreads as separate JPGs and numerous 1x1 px PNGs which seemed to have no actual function, so they can be deleted.

Any scriptable photo editor would shift the levels in the same way in all files. Unfortunately you do not use Photoshop where the script can be a recorded action which needs no programming skills.

Fast manual adjustment is possible in Paint.NET which remembers the last edit and offers the same settings automatically- simply open say ten spreads and apply the same levels adjustment to all. This is a sample screenshot from Paint.NET:

You need a way to combine the edited JPGs back to a PDF. A not so clever idea is to place them to an otherwise empty layout and print a PDF. I guess people with programming skills could write something better.

You probably have noticed that the text is selectable in the PDF. Adobe Reader either reads and recognizes the text images or OCR result is included in PDF as "OCR-layer". I tried Affinity Publisher (for first 10 spreads only). It found the texts, too. It shouldn't have OCR, so the text is included.

I changed the text color from transparent to black, deleted the JPG and I had 20 pages with readable and editable texts. The work = total a dozen clicks. Unfortunately the fonts were substituted (I haven't originals), but editing the substitution list is possible. Mostly A.Publisher offered Arial by default.

Here's a couple of sample screenshots from Affinity Publisher:

But is it reliable? It's not. It needs 100% proofreading and edits. I see here and there wrong letters. Not many, but there are errors. See footnote 5 on page 8. There are several times ch replaced by eh. It can be a ligature problem, but that's a guess only.

I tested also freeware LibreOffice. It tried different font replacements, but there seemed to be errors in same places than in Affinity Publisher. Font replacement attempts weren't all especially good, so there's more to fix than in Affinity Publisher.

Because Affinity suite programs all open PDFs quite equally but offer different edits easily, opening in Affinity Photo probably is the nearest (without Acrobat Pro) attempt which gives the wanted easy darkening. There the opened pages needed only this (+save as PDF):