Sharpen a PDF

How to sharpen a PDF

1. Pick a strength above. Gentle is selected to begin with, and it is usually enough.
2. Choose your PDF, or drop it on the page. It is processed in your browser and never uploaded.
3. Drag the slider to compare the original with the result. Zoom in on small text, because that is where sharpening shows.
4. If it is too weak, raise the Amount or choose a stronger strength, then press Apply new settings. If you see rough edges or grain, lower the Amount or raise Noise protection.
5. Download the new PDF. The original is left untouched.

What sharpening does here

The method is an unsharp mask on brightness. A blurred copy of the page is subtracted from the page to find the edges, and a controlled amount of that difference is added back, which steepens each edge.

• Halo limit. A pixel is never pushed lighter than the lightest pixel beside it, or darker than the darkest. That is what stops the bright and dark outlines that heavy sharpening usually leaves around letters.
• Noise protection. Small variations are ignored, so paper grain and compression noise are not amplified. Raise it if a scan looks gritty after sharpening.
• Colour is untouched. The same adjustment is applied to every channel, so a blue stamp stays blue and cream paper stays cream.
• Tone is untouched. Unlike the other tools here, Sharpen does not darken ink or whiten paper. Use contrast or the PDF enhancer if the page also needs that.

What sharpening cannot do

• It cannot add detail. If a scan was captured at too low a resolution, the missing detail is not recoverable, and sharpening only makes the existing edges look firmer.
• It cannot fix heavy blur, such as a camera that moved or was out of focus. Very strong sharpening on a badly blurred page produces rough, artificial edges rather than readable text.
• It cannot undo JPEG blockiness, and it will not make faint text darker.
• It does not read or re-type the text. To make a scan searchable, use OCR a scanned PDF.

Which pages get sharpened

Pages that contain images, including every scan, are rendered at 200 DPI, sharpened, and written back as images. Pages made only of text and vector graphics are already sharp at any zoom, so they are left untouched, and the result panel tells you when that happens.

The three strengths

• Gentle — a light touch with strong noise protection. Safe for most scans.
• Standard — noticeably crisper edges on soft scans.
• Strong — for very soft pages. Check the result for rough edges.
• Move a slider and the strength shows as Custom. Your numbers are used exactly as set.