The easiest way to compress scanned PDF files is to lower the resolution and quality of the page images, because a scan is simply a set of pictures. Open Compress PDF, move the slider, check that the smallest text is still readable in the preview, and download. Scans respond very well to compression, so large savings are normal.
Why scanned PDFs are heavier than normal PDFs
A PDF exported from a word processor stores letters as text. A page of text needs very little space. A scanned page stores a photograph of the paper instead, including the texture of the page, shadows, and every speck of dust.
Three choices made at scan time decide the size:
- Resolution. More dots per inch means a sharper image and a bigger file.
- Colour mode. Full colour is the heaviest, grayscale is lighter, and pure black and white is the lightest.
- Page count. Every page is another picture, including blank backs.
The good news is that most scans carry more detail than a reader needs. That spare detail is what compression removes.
How to compress a scanned PDF step by step
- Open the Compress PDF tool in your browser. It works on phones, tablets and computers.
- Add your scanned PDF. Keep the original file untouched in case you need it later.
- Set the slider to a moderate level to begin with.
- Look at the before and after size, then check the preview. Zoom in on the smallest print, any stamp and any signature.
- If everything is clear, try one step stronger. If letters start to break up or look smeared, go back one step.
- Download the compressed file to your device.
- Open it once more and read a few lines on the busiest page before you send it.
PDFNova processes files in your browser, so your documents are not uploaded to a server. This matters with scans, which are often certificates, statements and ID documents.
Which setting suits which kind of scan
| Document | What must stay clear | Suggested approach |
|---|---|---|
| Typed letters and forms | Body text | Stronger compression is usually fine |
| Contracts with small print | Fine print, signatures, initials | Moderate level, check every signed page |
| Certificates and transcripts | Seals, stamps, grades, serial numbers | Light to moderate level |
| Handwritten notes | Pencil and light ink strokes | Light level, faint writing fades quickly |
| ID cards and passports | Photo, numbers, machine-readable lines | Light level, keep colour |
| Receipts | Totals and dates | Moderate level, thermal print is often faint |
These are starting points, not rules. The preview on your own file is the final judge.
Get a smaller scan from the start
Compression fixes a heavy file after the fact. If you still have the paper, a better scan gives a smaller file and a cleaner result than squeezing a bad one.
With Camera Scanner you can scan with your phone or laptop camera. The browser asks for camera permission, the tool detects the page edges automatically, and you can adjust the corners by hand. Perspective correction straightens the page, and you can add several pages into one PDF.
For size, the filter is the most important choice:
- Black and white for typed text, forms and printed letters. Smallest files, strong contrast.
- Grayscale for pages with pencil, light stamps or photos where colour is not needed.
- Colour only when colour carries meaning, such as an ID photo, a coloured seal or a chart.
Good light matters too. A page photographed in dim light has noise and shadows, and noise is hard to compress. Place the paper on a dark, flat surface near a window and hold the phone parallel to it. Our guide on how to scan documents with your phone camera with no app needed covers the full technique.
Troubleshooting a scanned PDF that will not shrink enough
The text turns blurry before the file is small enough
The scan probably started at a low resolution or in poor light, so there is little spare detail to remove. Rescan the pages in black and white if you can. That often gives a smaller file than any amount of compression on a colour scan.
The file is still too big after strong compression
Look at the page count. Remove blank backs and duplicates with Delete Pages, then compress the original again. If the receiver accepts several files, splitting the document is another clean option.
The background looks grey and dirty
That is the paper itself, captured in colour or grayscale. A black and white filter at scan time turns the background plain white, which also makes the file lighter.
Stamps and signatures have faded
Light blue ink and faint stamps suffer first. Go back to the original and choose a lighter level, or rescan those pages in colour and keep the rest in black and white.
The size barely changed
The file may already have been compressed once. Compressing the same scan again and again gives small gains and growing damage. Always work from the earliest copy you have.
Should you run OCR before or after compressing?
OCR reads the letters in a scan and makes the text searchable. It works best on a sharp image, so run OCR PDF on the clean original scan if searchable text matters to you. Heavy compression first can lower recognition accuracy, especially for small print and for scripts such as Urdu Nastaliq, which are harder to recognise than printed English.
If you only need a small file to upload, you can skip OCR entirely.
Before you send: a short checklist
- All pages are present and in the right order.
- Names, numbers and dates are readable without zooming far in.
- No page is upside down or cut off at the edge.
- The size is under the limit shown on the upload page.
- The original scan is saved somewhere safe.
If the file is going to an application form, see how to reduce PDF size for job and university application portals. If you are doing all of this on a mobile, how to compress a PDF on your phone has tips for small screens.
Frequently asked questions
How do I reduce the size of a scanned PDF without losing readability?
Compress in small steps and check the smallest text in the preview each time. Stop one step before letters start to smear. For text documents, rescanning in black and white gives the best mix of size and clarity.
Why is my scanned PDF so large?
Each page is a full picture, often in colour and at high resolution. A few pages of scanned paper can outweigh a long text document.
Can I compress a scanned PDF on my phone?
Yes. The tool runs in a mobile browser. Add the file, set the slider, check the preview and download the result to your phone.
Does compressing a scanned PDF remove pages or content?
No. Compression lowers image resolution and quality. The page count and layout stay the same, though very fine detail can soften at strong settings.
A scanned file does not have to be a heavy file. Scan in the lightest filter that suits the document, then finish the job with Compress PDF and check the result before you send it.