Contrast and brightness
F = 259 × (C + 255) ÷ (255 × (259 − C)); v′ = F × (v − 128) + 128 + 2.55 × B
Contrast pushes values away from mid-grey 128; brightness adds the same offset to every component.
Adjust PDF contrast and colors
Visually adjust PDF contrast, brightness and saturation, or convert its pages to grayscale, entirely locally.
- Export
PDF document
No PDF loaded
Drop a PDF here
Import a PDF, choose an action, edit the document directly in the preview, then download a new copy.
Need more editing options?
Use multiple editing tools in one workspace, with your open file and its current edits.
Open the full editorWas this tool useful?
How does this tool work?
Adjusting a PDF’s contrast and colours makes a dull or poorly scanned document easier to read: strengthen a faint scan, lighten a page that is too dark, tone down or boost colours, or produce a greyscale version for printing. The same settings apply to every page, with an instant preview, directly in your browser.
The Adjust colors button offers four settings: Contrast and Brightness from −100 to +100 (contrast at +20 by default), Saturation from −100 to +100, and a Grayscale box. An Output quality slider, from 45 to 95% (82% by default), sets the compression of the pages produced.
So that the correction applies to everything visible, scans included, each page is first drawn as an image (scale 1.35, about 97 dots per inch), then every pixel is recalculated with the formulas below. The corrected pages are finally assembled into a new PDF at the original page size, as JPEG images.
This method has a consequence: text is no longer selectable or searchable in the resulting file, and a very sharp document may lose a little detail. Keep the original if you need to copy text, or run the result through OCR afterwards.
How to use this tool
Import the PDF
The document opens in the editor with the Adjust colors button.
Set the colours
Adjust contrast, brightness and saturation, or check Grayscale, while watching the preview.
Apply and download
Choose the output quality, then download the corrected version.
Formulas applied to each pixel
The red, green and blue components of each pixel, written v and ranging from 0 to 255, are recalculated with these formulas and then clamped to 0–255.
Luminance (greyscale)
Y = 0.2126 × R + 0.7152 × G + 0.0722 × B
Green weighs most because the eye is most sensitive to it; in greyscale, all three components take the value Y.
Saturation
v′ = Y + (v − Y) × (1 + S ÷ 100)
Negative saturation pulls each component towards grey Y, positive saturation pushes it away.
- C, B, S
- contrast, brightness and saturation settings, from −100 to +100
- v, v′
- component before and after correction (0 to 255)
- R, G, B
- red, green and blue components of the pixel
- Y
- luminance of the pixel
Use cases
Make a faint scan readable
Boost the contrast of a document photocopied several times or scanned too light.
Prepare black-and-white printing
Convert to greyscale to check the result before printing and avoid surprises.
Even out pages
Apply the same correction to a document assembled from scans made in different conditions.
Examples
Contrast +20 on two greys
F = 259 × 275 ÷ (255 × 239) ≈ 1.1687; 1.1687 × (100 − 128) + 128 ≈ 95 and 1.1687 × (200 − 128) + 128 ≈ 212. The gap between the two tones grows from 100 to 117, so text stands out better from the background.
Input
Contrast +20, brightness 0; a dark grey pixel (100) and a light grey pixel (200)Result
The dark grey becomes 95 and the light grey 212Tips and best practices
- Start with contrast at +20 and increase in small steps: beyond +60, light greys disappear.
- For a yellowed background, combine a slight brightness increase with lower saturation.
- Raise output quality to 90% for documents with small print.
- Run OCR after the correction: a well-contrasted scan is recognised better.
Frequently asked questions
Why is my text no longer selectable?
Because the pages are recalculated as images to apply the correction everywhere. Run the result through OCR PDF if you need searchable text.
Does the page size change?
No. Each page keeps its original dimensions; only the image making it up is recalculated.
What exactly does Grayscale do?
It replaces each colour with its luminance, calculated with the coefficients 0.2126, 0.7152 and 0.0722 applied to red, green and blue.
Can I correct just one page?
No, the settings apply to the whole document. Extract the page, correct it, then reassemble if needed.
Will the file be heavier?
It depends: for a scan it often stays similar; for a vector text PDF it usually grows. Output quality lets you adjust that trade-off.
Are my pages sent away for processing?
No. Rendering, pixel calculations and rebuilding the PDF happen in your browser.
Related tools
Keep going with these tools
Tools similar or complementary to this one.
Works well with
Recognize text in a scanned PDF and create a searchable PDF directly in your browser.
PDF & documentsUse this tool
Reduce PDF file size locally with lossless optimization or stronger visual compression.
PDF & documentsFeaturedUse this tool
Automatically detect and remove blank PDF pages directly in your browser.
PDF & documentsUse this tool
Read next
GuideBest practicesBeginner
How to clean and OCR a scanned PDF
Clean a scanned PDF before OCR to remove unnecessary pages and create a more useful searchable document.
6 September 20262 min
TutorialBest practicesBeginner
Fill and flatten a PDF form before sharing it
Learn how to fill interactive PDF fields and flatten them into a finalized document for sharing.
10 September 20261 min
TutorialBest practicesBeginner
How to securely redact a PDF before sharing it
Learn why covering text is not enough, how secure PDF redaction removes sensitive information and why sanitization matters before sharing.
10 September 20262 min
TutorialBest practicesBeginner
Prepare a PDF before sending it: merge, reorder, compress and convert
A practical workflow for checking, merging, reordering, sanitizing, compressing and protecting a final PDF before sending it.
30 August 20263 min