Fix a Scanned PDF That Is Too Dark

How to fix a scanned PDF that is too dark

Open the PDF in the tool above and look at the preview: the page is re-rendered with a new white point. The tool reads the brightest ten percent of pixels on each page — that is the paper, whatever shade of gray or yellow the scanner turned it into — and scales every colour channel until that paper shade becomes pure white. Because the scaling is done per channel, a yellow or blue colour cast disappears at the same time. The text, which was lifted along with the paper, is then pushed back towards black with the Darken ink control, so you get black letters on a white page instead of gray letters on a light-gray page. Move the two sliders while watching the preview, click Process, and download. Every page is processed with the same settings, and nothing is uploaded: the PDF is rendered, corrected and rebuilt inside your browser.

Step by step

1. Drop the dark PDF onto the upload area, or click Choose File.
2. Pick the page you want to preview if the first page is not representative (a cover, for example).
3. Leave Whiten paper at 100 for a scan that is uniformly dark. Lower it if the page starts to look washed out.
4. Raise Darken ink until thin strokes are solid black; stop before letters start to fill in.
5. Tick Convert to grayscale if the document has no meaningful colour — it removes speckled colour noise from cheap scanners.
6. Click Process, then download the brightened PDF.

Why scans come out dark in the first place

A dark scan almost always comes from exposure, not from the paper. Flatbed scanners and multifunction printers meter the whole glass; if the lid is open or the document is smaller than the glass, the dark surroundings pull the exposure down and the page is recorded as mid-gray. Phone cameras do the opposite thing for the same reason: their metering assumes a scene should average to a medium tone, so a sheet of white paper is deliberately underexposed to gray. Photocopies of photocopies build up toner in the background with every generation, and thermal receipts, newsprint and recycled paper are simply not white to begin with. Finally, some scanner drivers save in a "photo" colour profile that preserves the gray faithfully instead of applying the "document" correction that would have snapped it to white.

Whatever the cause, the result is the same in the file: the paper sits somewhere between 90 and 180 on the 0–255 brightness scale instead of near 255, and the text sits at 20–60 instead of near 0. The distance between them — the contrast — is smaller than it should be, and the whole range is shifted down.

Why a brightness slider is the wrong fix

The obvious reaction is to turn up brightness, but brightness adds the same amount to every pixel. If the paper is at 130 and the text at 40, adding 100 gives you paper at 230 and text at 140: a light page with pale gray letters that is harder to read than the original. Contrast helps a little, but it stretches around the middle of the range and clips whichever end is closer.

What a dark scan needs is a change of white point: multiply instead of add. Scaling by 255 ÷ 130 takes the paper to 255 and the text only to 78, so the gap between them grows in proportion. The tool then applies a gamma curve controlled by Darken ink, which leaves white alone and pulls the mid-tones down, taking that 78 back towards black. That is the difference between this page and the general-purpose contrast and brightness editor: that tool gives you the raw sliders; this one measures the page and chooses the multiplication for you, per colour channel.

Dark scan, faint scan, shadowed scan: which tool

These look similar in a thumbnail and need different corrections. A dark scan has gray paper and dark text everywhere on the page — the whole image is shifted down, and this page fixes it. A faint scan has white paper but light-gray text — the range is compressed at the top, and the answer is more contrast, which the adjust contrast tool does best. A shadowed scan is bright on one side and dark on the other, or has a gradient near the spine of a book; no single white point can fix that, so use the shadow removal tool, which estimates the paper shade separately for every region. If the page is also crooked, straighten it first with the deskew tool; a straight, white page is also what OCR reads most accurately.

What happens to photos and colour graphics

The white point is measured on the whole page, so a page that is mostly text with a small photo is handled well: the paper becomes white and the photo is brightened by the same factor, which usually looks natural because it was underexposed by the same amount. A page that is mostly a dark photograph is a different case — the "brightest ten percent" is then part of the photo, and lifting it to white will wash the image out. For those pages lower Whiten paper, or process the text pages separately after cutting the file with the split tool.

Getting a bright scan next time

Close the scanner lid, or lay a sheet of white paper behind a small document so the glass is not exposed. On a phone, use a document-scanning mode rather than the plain camera: the VoxScan scanner app for iOS and Android detects the page edges, corrects exposure and colour at capture time and exports straight to PDF, so the file does not need fixing afterwards. Avoid the flash, which creates a hot spot in the middle and a dark rim, and shoot in even daylight instead.

Frequently asked questions

Does this work on multi-page PDFs?

Yes. The preview shows one page so you can tune the sliders, and Process applies the same settings to every page in the file. If pages vary a lot in darkness, split the file first and process the batches separately.

Why is my text still gray after brightening?

The paper has been taken to white but the ink was lifted with it. Raise Darken ink: it applies a gamma curve that pulls mid-tones down without touching white. Values around 30–60 suit most scans.

Will it change the page size or layout?

No. Each page is rendered at high resolution, corrected pixel by pixel, and placed back at its original dimensions.

Does the output stay searchable?

The corrected pages are stored as images, so an existing text layer is not carried over. Run the result through the OCR tool to add a searchable layer — recognition is more accurate on the brightened version.

Is my PDF uploaded to a server?

No. Rendering, correction and rebuilding all happen in your browser with JavaScript; the file never leaves your device.

Can it fix a photo of a page taken with a phone?

Yes, if the lighting is roughly even. A phone photo that is dark on one side needs the shadow removal tool instead, which corrects each region separately.