How to darken a PDF: six methods compared

1. Fix it at the source

If you have the original document, change the text colour to black and export again. It is the cleanest fix, because nothing is processed afterwards. It only works when you have the source and the PDF came from a document rather than a scan.

2. Scan it again with better settings

If the problem is a faint scan and you still have the paper, rescanning beats any processing. Scan at 300 DPI in greyscale rather than black and white, and turn on background removal if your scanner offers it. Avoid economy or “fast” profiles. This is the best result for a physical document, but it needs the paper.

3. Raise the printer’s density

Many printers have a density or darkness setting, and economy and draft modes lighten everything on purpose. Turning those off helps every document, but it is a blunt tool: it darkens the background of a grey scan as much as the text, and it only applies to that printer.

4. Change the text colour in a PDF editor

For a digital PDF whose text is grey, a PDF editor that can edit live text can set it to black. It works on real text only, it is manual, and it can disturb the layout if fonts are substituted. It does nothing for scanned pages.

5. Use levels or curves in an image editor

For a scan, an image editor’s levels or curves can pull ink towards black, but the obvious slider is the wrong one. On a faint scan, raising brightness lightens the ink along with the paper and makes it worse; measured on this site’s faded demo scan, a plain brightness boost of 30 narrowed the gap between ink and paper from 44 to 38. Contrast stretched about the mid-grey did not help either: it narrowed the gap to 39, and a stronger version to 9. See brightness versus contrast. Levels done properly, with the black point set at the ink and the white point at the paper, does work, but it is per page and fiddly.

6. Use a tool that measures the page

A tool that measures how far the ink is from the paper can set the black and white points itself and apply them to every page. Darken PDF does this in your browser: on a text page it rewrites the text colour, and on a scan it re-tones the image. On the same demo scan, Strong contrast moved the ink from 187 to 81 and the paper to 255, a gap of 174. You see a before/after preview before you download, and your original is never modified.

Which one should you use?

• You have the source document: method 1.
• You have the paper and the scan is faint: method 2, then method 6 for what remains.
• Everything prints faint: method 3, plus a check of the printer settings.
• A digital PDF with a few grey passages: method 4 or 6.
• A scanned or photocopied PDF and no paper: method 6.