How to Compress a Scanned PDF

To compress a scanned PDF, use a tool that re-encodes the full-page images inside it at a lower quality or resolution — that is where nearly all the file's weight lives. Because a scan is basically a stack of page images, scanned PDFs often shrink the most, and with an in-browser tool the whole thing happens on your device, so the file is never uploaded.

Last updated: July 2026

That single idea — a scanned page is an image — is the key to why scans get so heavy and why they compress so well. Below we explain where the size comes from, how to compress a scanned PDF step by step, and how to squeeze a scan that is still too big without sending it to a stranger's server.

Why are scanned PDFs so large?

A scanned PDF is not really a "document" in the way a typed one is. When a scanner or a phone captures a page, it takes a photograph of that page and drops the picture inside a PDF wrapper. So a 20-page scan is, under the hood, 20 full-page photographs stacked in order. Every speck of paper texture, every shadow, and every bit of color is stored as image data.

That is completely different from a PDF you export from a word processor, where the text is stored as lightweight vectors — instructions like "draw this letter here" that take almost no space. Vector text is tiny. Full-page images are not. This is exactly why the same content can be 80 KB as a typed PDF and 20 MB as a scan of the printed version.

A few things make scans especially bloated:

• Resolution. Scanners often default to 300, 400, or 600 DPI. Each step up multiplies the pixel count, and most scans are captured at a far higher resolution than anyone actually needs to read them.
• Color depth. A black-and-white contract scanned in full color carries three color channels it does not need.
• Weak compression. Many scanners embed the page images with little or no compression, leaving obvious savings on the table.

The good news: because all that weight is in the images, a scanned PDF is the ideal candidate for compression. A tool that re-encodes those embedded page images can often shrink a scan dramatically — far more than it could ever shrink a mostly-text PDF.

How to compress a scanned PDF

Shrinking a scan takes about a minute. With MumenLabs Compress PDF, the whole process runs inside your browser, so your scanned pages never leave your device:

1. Open the tool and sign in. A free MumenLabs account keeps your workspace tied to you — not your files. Everything after this step happens locally on your machine.
2. Add your scanned PDF. Drag it in or select it from your computer or phone. This reads the file locally; nothing is uploaded, so there is no progress bar to wait on.
3. Choose a compression strength. Pick Light for a gentle trim, Recommended for a balanced shrink, or Strong for the smallest file. For an oversized scan, Strong is usually the right starting point.
4. Or go Custom for oversized scans. Open Custom mode and lower the maximum image resolution — on a scan captured at a needlessly high DPI, this is the single biggest lever you have. Pair it with the image-quality slider to dial in the exact size-versus-clarity balance you want.
5. Compress and download. Click compress and your scan is re-encoded locally in seconds. You will see the real before-and-after size and the exact percentage saved, then download the smaller file straight to your device.

If you have several separate scans, you can drop them all in at once and download them individually or together as a ZIP — there are no file-count or task limits.

Which setting shrinks a scanned PDF the most?

Lowering the maximum image resolution almost always beats everything else on a scan. Because file size scales with the pixel count, halving the resolution of an oversized scan can cut the size enormously while still leaving pages perfectly readable on screen. Most scans are captured at a far higher DPI than a screen or a normal printout ever uses, so there is usually a lot of headroom to cut.

The image-quality slider is the second lever. It controls how aggressively each page image is compressed. Turning it down introduces more compression but keeps the pixel dimensions the same, which is useful when a scan's resolution is already reasonable and you just want a lighter file.

The built-in presets bundle both levers for you:

• Light — minimal resolution and quality reduction, for when a scan is only slightly too big.
• Recommended — a balanced cut that noticeably shrinks most scans while keeping them clearly legible.
• Strong — the most aggressive preset, ideal when a scanned PDF is too big for an email or upload limit and you need the smallest possible file.

Whichever route you take, the tool never hands you a larger file. If a scan is already lean, it keeps your original and labels it already optimized, so you are never worse off than when you started.

Does compressing a scan blur the text?

The page images do get re-encoded, so if you push resolution and quality very low, the scanned page will look softer — that is the trade-off you are choosing with the sliders. The trick is to cut only as far as your use needs: a scan headed for email can be squeezed hard, while one you want to keep crisp for printing should be squeezed gently. Because the tool shows the exact percentage saved, you can find the point where the size is small enough but the page still reads cleanly.

There is an important nuance for scans that have already been run through OCR. If your PDF has a real text layer on top of the scanned image, that text is stored as sharp, selectable vectors — and it stays that way. The compression only touches the page images; it leaves the OCR text layer untouched, so words you can select and search stay just as selectable and searchable afterward.

Image-only recompression vs. re-rasterizing the whole page

Not every "compress" tool shrinks a scan the same way. The distinction matters:

	Image-only recompression	Re-rasterizing the whole page
What it does	Re-encodes the embedded page images at a chosen quality/resolution	Flattens every page — including any real text — into one new image
A real OCR text layer	Kept as sharp, selectable vectors	Destroyed; folded into a flat picture
Best for	Scans, where the images are the weight	Rarely the right call; loses searchable text
Result	Smaller scan that still works like a document	Smaller file, but selectable text is gone

MumenLabs Compress PDF uses the first approach: it targets the heavy embedded images that make a scan large and leaves any genuine text layer alone. For a scan, that is exactly what you want — the images are precisely the weight you are trying to lose.

Combining several scans before you compress

Scanned paperwork often arrives in pieces — a form, its attachments, a signature page — each as its own PDF. If you want a single tidy file, join them first and compress once. You can stitch them together with Merge PDF and then run the combined document through the compressor, so one lean file covers the whole bundle instead of a scattered handful of heavy ones.

Why compress scans in your browser?

Scans are frequently the most sensitive documents people handle — signed contracts, tax returns, IDs and passports, medical records, payroll. Sending any of those to an unknown server just to make them smaller is a risk you should not have to take, and for regulated work it can be an outright compliance or NDA violation.

An in-browser tool removes that risk entirely. Your scan is read into your browser's memory, re-encoded there, and written back out as a smaller PDF — all on your own device. Nothing is transmitted. You can prove it, too: once the page has loaded, switch off your internet and compress anyway. It still works, because there is nothing to send. A tool that uploaded your files could not function offline; this one can.

The output is clean, too: no watermark, no logo, and no file-size or task limits. For the fundamentals of keeping quality high while cutting size, see our guide on how to compress a PDF without losing quality, and for more on the privacy angle, compress PDF without uploading.

Frequently asked questions

Does compressing a scanned PDF make it searchable?

No. This tool compresses; it does not OCR. It re-encodes the page images to shrink the file, but it does not read the text on the page or add a searchable text layer. If your scan is image-only to begin with, it stays image-only afterward — just smaller. If it already has a text layer from a previous OCR step, that layer is preserved and stays selectable, but no new one is created.

How much smaller will my scanned PDF get?

Usually a lot. Because a scan is essentially full-page images, and images are where nearly all of a PDF's weight lives, scanned PDFs typically shrink far more than mostly-text files do. The exact amount depends on the original resolution and quality — an oversized high-DPI scan has the most to give. The tool shows the real before-and-after size and the exact percentage saved, so there is no guesswork.

My scanned PDF is still too big — what should I do?

Open Custom mode and lower the maximum image resolution, then reduce the image quality a step or two. Resolution is the biggest lever on a scan because file size scales with pixel count, so trimming an over-scanned document there can cut the size dramatically while keeping the pages readable on screen. Try Strong first, then move to Custom if you need to reduce the scanned document further.

Do my scanned files leave my device when I compress them?

No. The entire compression runs locally in your browser with MumenLabs Compress PDF. Your scans are read, re-encoded, and saved entirely on your own device — never uploaded to our servers or anyone else's. You can confirm it by disconnecting from the internet before compressing: it still works, which proves nothing is being sent anywhere.

Compress your scanned PDFs privately

A scanned PDF is heavy because every page is an image — and that is exactly the weight a good compressor targets, which is why scans so often shrink the most. To compress a scanned PDF without handing your documents to a stranger's server, use MumenLabs Compress PDF: choose a strength, drop the resolution on oversized scans, see the real size saved, and download a smaller file — all 100% in your browser, free, with no watermark and no limits. Your scans stay exactly where they belong: on your device.