How do I make a grey, speckled scan readable?

Your scan looks like a photocopy of a photocopy: grey where it should be white, soft where it should be sharp, and dotted with specks that were never on the paper. You can read it if you try, but you wouldn't send it to a client.

Why it looks like that

The scanner didn't fail. It recorded exactly what it saw, including things your eye ignores when you look at the paper itself.

Paper is not white. It is pale grey or cream, and under a scanner lamp it stays pale grey. Your brain corrects for that when you hold the page. The file can't.

Ink is not black. Printer toner is dark grey, and a copy of a copy is greyer still. So a scan often has dark-grey text on light-grey paper. The tones are close, so it looks washed out.

Specks are real. Dust on the glass, paper fibres, toner scatter. At scanning resolution they become visible dots.

Making the image brighter won't fix it. Brightness lightens the text and the paper together, so the gap between them stays the same. You want to push them apart.

What each control does

• Contrast widens the gap between light and dark. This is the one that fixes a washed-out page, so try it first.
• Black and white goes all the way: every pixel becomes one or the other. Grey paper becomes white, grey text becomes black, and most specks disappear with the grey around them. On an ordinary printed document this is usually the best thing you can do.
• Greyscale keeps the tones but drops the colour. Useful when a page has a coloured background you want gone but contains photographs you do not want reduced to dots.
• Sharpen tightens soft edges. Helpful on a slightly out-of-focus phone photograph; on an already-crisp scan it mostly amplifies the speckle.
• Brightness is for pages that are too dark or too light overall. Touch it last.

Enhance Scan applies these to every page of a PDF, or to a single photograph.

The thing nobody mentions

Cleaning a PDF turns its pages into pictures.

The changes above (contrast, black and white, speck removal) work on images, so each page has to become an image first. What comes out is a PDF of cleaned pictures.

For a scan this costs you nothing. A scan was already a picture of words. A cleaner picture of the same words is pure gain, and recognition works better on it afterwards.

For a document that had real text in it, you lose something: the words stay readable but can no longer be selected, searched or copied. That is why you get a warning when the file you uploaded still has a text layer, so you can decide before it happens.

Clean first, then recognise

If your goal is to get the words out, or to edit them, do the cleaning first.

Recognition starts by deciding which pixels are ink and which are paper. On a grey, speckled page it gets that wrong, and every later step carries the mistake. On a black-and-white page the answer is already clear.

The order that works: enhance, check it looks right, then run OCR PDF or extract the text.

It usually gets smaller too

A black-and-white page compresses far better than a grey one, because there are two values to store instead of two hundred and fifty-six. A cleaned bundle of scans is often a fraction of the original size. That is a nice extra. If size is what you care about, Compress PDF is made for that.