Make a scanned PDF darker

Why scans come out faint

• The original was already faint — a carbon copy, a thermal receipt, a third-generation photocopy.
• The scanner exposed for the paper rather than the ink, which is the safe default and the reason so many scans look washed out.
• Automatic background removal was off, so the paper colour was captured as grey rather than dropped to white.
• A phone camera added its own shadow, so one corner of the page is noticeably darker than the other.
• The scan was saved as a low-quality JPEG, which softens edges before you ever open the file.

Uneven lighting is handled separately

A single global adjustment cannot fix a page that is bright in the middle and shadowed at the edges: whatever setting rescues the shadow blows out the middle.

Instead, the paper level is estimated across the page on a grid, smoothed, and used to normalise each region against its own local background. Shadows flatten out, the paper reads as one consistent white, and the ink keeps its relative darkness. That is the same idea a good scanner driver applies at capture time — applied after the fact.

If you can, rescan instead

Software can only work with what the scan captured. If the original document is still available, a better scan beats any amount of post-processing.

• Scan at 300 DPI for text. 600 DPI helps only for very small print.
• Choose Greyscale rather than Black & White for anything faint — thresholding at capture time throws detail away permanently.
• Turn on background removal or descreening if your scanner offers it.
• For phone scans, use diffuse light from the side, not a direct overhead lamp, and keep the phone parallel to the page.