How to darken a scanned PDF

First, work out which kind you have

Open the PDF and try to select a line of text with the mouse. If a neat highlight follows the words, the page is digital text. If nothing happens, or a rectangle sweeps across the whole page, it is a scan.

A third case is common: a scan with an invisible OCR text layer sitting on top. Selection works, but the visible page is still an image — and it is the image that needs darkening.

Why a global brightness change makes scans worse

The instinctive move is to open an image editor and drag brightness down. On a scan this rarely works, because the paper goes dark along with the ink and legibility drops.

The second instinct — contrast — is closer, but a single global contrast setting cannot cope with a page that is unevenly lit. Whatever value rescues the shadowed corner destroys the bright centre.

What actually works

1. Estimate the paper locally. Sample the lightest values across a grid over the page and smooth them into a map of "what the paper looks like here". Shadows, gradients and grey casts all become part of that map.
2. Normalise against it. Dividing each pixel by its local paper level flattens the lighting: the paper reads as one consistent white and the ink keeps its relative darkness.
3. Despeckle the light areas only. A small median filter applied where the page is close to white removes dust without rounding the corners of small text.
4. Apply a measured tone curve. Put the black point at the measured ink level and the white point at the measured paper level, and the ink lands near black and the paper near white.
5. Sharpen last. A mild unsharp mask restores the edge definition that the scanner optics and JPEG compression softened. Doing this before the curve would amplify noise instead.

When a rescan beats processing

Software can only work with what the scan captured. If you still have the paper, rescanning is often faster than fighting a bad file.

• Scan at 300 DPI for ordinary text; 600 DPI only for very small print.
• Choose Greyscale, not Black & White. Thresholding at capture time throws away the mid-tones you would need to recover faint strokes.
• Enable background removal or descreening if the driver offers it.
• For a phone scan, use diffuse side light and hold the phone parallel to the page. Overhead lamps produce exactly the shadow gradient that is hardest to fix.

Keeping signatures and stamps

The tempting shortcut for scans is a hard threshold: everything darker than X becomes black, everything else white. It produces impressively crisp text and it destroys signatures, stamps and any half-tone content, because those live in exactly the mid-tones it discards.

A tone curve keeps the greys and simply spreads them out, which is why the scanned-PDF darkener uses one by default and treats coloured content more gently still.