How to make a scanned PDF smaller and clearer

A scanned PDF is a stack of pictures, one per page. To make it smaller, give each picture fewer pixels and fewer colours. To make it clearer, give it a white background and dark, sharp text. For ordinary text pages, a black-and-white scan at 150 to 300 DPI does both at once, which is why smaller and clearer are not opposites here.

What you can't do is add detail afterwards. If the original photo was blurred or taken in poor light, no setting will sharpen it. Scan the page again; it takes less time than fighting the file.

Why scanned PDFs are so big

Three things decide the size of a scanned page.

• Pixels. A 12 megapixel phone photo is about 4000 × 3000 pixels. Spread over an A4 page, that is roughly 340 DPI, more than most forms need.
• Colour. A full-colour image carries three channels of information for every pixel. A grey one carries a single channel.
• Background noise. Grey paper, shadows, creases and the table around the page are all detail the file has to store. A flat white background compresses to almost nothing.

Here is what resolution means in pixels for an A4 page:

Going from 300 to 150 DPI cuts the pixel count to a quarter. That one change often does more than any quality slider.

Scan it well the first time

The cleanest small file comes from a good capture, not from heavy compression later. Our Scan to PDF tool works in your phone's browser with no app: take a photo of each page, drag the four corners to the edges of the paper, pick a look and press Create PDF.

1. Use daylight or a bright, even light. Stand so your hand and phone don't throw a shadow on the page. Avoid the flash on glossy paper.
2. Lay the page flat on a dark, plain surface. The contrast makes the paper's edges easy to see when you set the corners.
3. Hold the phone directly above the page and fill the frame. Tap to focus on the text and keep still for a second.
4. Drag the corners to the paper's edges. This straightens a tilted photo into a flat rectangle and removes the table, which also removes wasted file size.
5. Choose the look. Black & white document turns shadows and grey paper white and keeps the text black. Use it for typed and printed pages.
6. Check page order, then create the PDF. You can reorder, rotate and delete pages first, and add up to 50 photos.

Which look to pick

Black and white is strict: every part of the page ends up either dark or light. That is ideal for text and poor for faces, so don't use it on your Aadhaar or passport photo page.

Shrinking a scanned PDF you already have

If the scan is already a PDF, use our Compress PDF tool. It has two modes, and for scans both are worth knowing.

• Keep text re-compresses the pictures inside the PDF. Balanced limits each image to 2000 pixels on the long edge at JPEG quality 75%; Strong uses 1400 pixels at 60%. On an A4 page that works out to about 170 DPI and 120 DPI. Start with Balanced.
• Smallest size redraws every page as an image and lets you set a target: 100 KB, 200 KB, 500 KB, 1 MB or your own figure. It tries resolutions from 150 DPI down to 60 DPI and keeps the sharpest result that fits. It warns you when it has to go below 100 DPI, because small text may become hard to read.

A scan has no selectable text to lose, so the usual drawback of Smallest size doesn't apply. The real limit is page count. A target is shared by every page, so a 10-page scan squeezed into 200 KB leaves about 20 KB per page, and that is rarely readable. If the tool can't reach the target it gives you the smallest result and says so.

When a multi-page file won't fit:

• Remove pages the form doesn't ask for, such as blank backs and covering letters.
• Ask whether the portal accepts separate uploads for separate documents.
• Rescan in black and white. Clean pages shrink much further than grey, shadowy ones.

Making an old colour scan black and white

For a PDF that was scanned in colour and has a grey or yellowish background, our Grayscale PDF tool can convert every page to grayscale or pure black and white at 150 or 300 DPI. Pick Black & white and 150 DPI for text pages, and look at the preview of page 1 before downloading.

Two cautions. The file size can go up as well as down, especially at 300 DPI, so compress afterwards if you need to hit a limit. And if a page has uneven lighting, pure black and white can turn the darker corner into a black patch. Grayscale is the safer choice for those pages, or rescan.

What official portals ask for

Requirements differ, so read the instructions beside the upload button. As one official example, the Income Tax e-filing portal's tips on scanning say to scan to PDF, at 300 DPI, in black and white only, on A4 or Letter size, from the original document and not from a photocopy or fax. It also says not to upload password-protected files.

That is a fair default for any government or bank upload when no other instruction is given. If the portal also has a tight size limit, scan in black and white first and compress second.

Common problems and fixes

• Text looks fuzzy after compression. The resolution went too low. Raise the target size, or use Keep text on Balanced.
• Dark shadow across one side. Rescan near a window or under even light. Black & white document mode cleans mild shadows, not a heavy one.
• Letters look broken or thin. The page was too faint for black and white. Use grayscale.
• The page is tilted or shows the table. Set the four corners again in Scan to PDF, or crop the page.
• The file is small enough but the portal still rejects it. Check the format it wants, the page size and whether it refuses password-protected files.
• You need to search or copy the text. Our scan tool doesn't do text recognition (OCR). The PDF holds images of the pages, so the text can't be selected.

Before you upload, open the final file and zoom in to 100% on the smallest print: a roll number, a date, an account number. If you can read it comfortably, the person checking it can too.

Questions people ask

What DPI should I scan documents at?

Follow the portal if it names one. Otherwise 150 DPI is enough for normal print read on screen, 200 DPI is a good middle for forms and small print, and 300 DPI suits fine print or anything that will be printed again.

Is black and white always smaller than colour?

For text pages it usually is, because a clean white background and solid black text leave very little to store. It isn't a promise for every file, and converting an existing PDF can occasionally make it bigger. Compare the sizes before you delete anything.

Can I make a blurry scan clearer?

Only a little. Black and white can lift faint grey text off a grey background, but it can't restore letters that are out of focus. Rescan the page.

Will compressing a scan reduce how it prints?

At 150 DPI and above, text pages print acceptably. Below 100 DPI small text gets rough. Keep your original scan so you can compress again at a different setting.

Are my scans uploaded when I use these tools?

No. Scanning, compressing and converting all run in your browser, and your photos and PDFs stay on your device.

Sources

• Income Tax Department: Tips on scanning documents