How to make PDF text darker

The problem with rasterising everything

The simplest way to darken any PDF is to render every page to a bitmap, adjust the pixels and write the bitmaps back. It works on everything, which is why so many tools do it.

It also costs you a great deal: text stops being text, so it is no longer selectable or searchable, it goes soft when zoomed, screen readers can no longer read it, and the file often grows several times larger.

For a page that was genuinely a scan, none of that matters — it was already an image. For a page of real text, all of it matters.

Darkening digital text without rasterising it

A PDF page stores its content as a stream of operators. Colour is set by specific instructions — a grey value, an RGB triple, a CMYK quadruple — and every subsequent drawing operation uses the colour most recently set.

So a page can be darkened by rewriting those colour operators and leaving everything else byte-for-byte identical. A 55% grey becomes near-black; a light grey background rectangle becomes white; a coloured logo is darkened more gently so it keeps its hue. The glyphs, fonts, positions, links and structure are untouched.

The result is a page that prints darker and is still, in every other respect, the document you started with.

When the text is already black

Sometimes zooming in shows genuinely black text that still prints faint. Then the problem is not colour but weight: a hairline font lays down very little toner, and screen anti-aliasing has been quietly thickening it for you all along.

The fix is to outline the glyphs — draw each character with a stroke as well as a fill, so every stroke gains a fraction of a point of width. That is what the Thicken thin text option does, and it is the only thing that helps in this case, because there is no darker colour than black to move to.

Keep it subtle. Too much outlining fills in the counters of small letters and makes 8pt text look like a smudge.

What about the scanned case?

There, darkening the text means separating ink from paper across the whole image: estimate the paper level locally, normalise the lighting, then map the measured ink towards black. Making a scanned PDF darker covers that path in detail.

A document can of course contain both kinds of page, which is why each page is classified and handled on its own terms rather than by one document-wide decision.