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Adjust PDF Colours

Brightness, contrast and saturation across a document. Free, and runs entirely in your browser — nothing you enter is uploaded, stored or sent to a server.

About Adjust PDF Colours

Brightness, contrast and saturation across a whole document. The case this exists for is the scan that came out washed out, grey and unreadable — a little contrast and a little brightness recovers a surprising amount.

The three are applied in order: brightness shifts every channel, contrast pivots around mid-grey, and saturation mixes each channel towards the pixel's own luminance. A saturation of -100 is exactly the greyscale conversion rather than something close to it — the same luminance weighting, the same result.

The contrast curve steepens sharply as it approaches its limit, which is why +20 does much more than +10. Move it in small steps and check the preview.

Where it helps

  • Three controls that do what they say, applied in a predictable order.
  • Saturation at -100 is exactly the greyscale conversion, so there is no discrepancy between the two tools.
  • The preview shows the first pages of the actual result rather than an approximation.
  • The document never leaves your browser.

Limits

  • The text becomes an image and can no longer be searched or selected.
  • Adjustments are global: a document whose pages were scanned at different exposures needs to be split and treated separately.
  • Pushing contrast hard clips the extremes — detail in the darkest and lightest areas is gone rather than compressed.
  • There is no auto-level or per-page analysis; the numbers are yours to choose.
  • Links, bookmarks and form fields do not survive the rebuild.

Frequently asked questions

Which should I move first?

Contrast, in steps of ten, until the text is properly black. Then brightness if the paper is grey. Saturation last, and only if the colours are wrong rather than the exposure.

Why does contrast do so much more at higher values?

Because the curve steepens as it approaches the limit — that is the standard contrast formula, and it is why +60 looks nothing like three times +20.

Can it fix a page that is half in shadow?

No. Everything here is applied uniformly to every pixel. Uneven lighting needs a local correction — the kind of adaptive thresholding a scanning app does — which is a different tool.

Is saturation -100 the same as the greyscale tool?

Exactly the same, computed by the same luminance weighting. Use whichever tool you found first.

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