How to compress a PDF without wrecking it
A scan compresses by ninety percent, a text document barely at all, and knowing which you have saves a lot of wasted effort. Here is what actually makes PDFs large.
Why PDFs get so large
A ten page text document is often under 200 KB. A ten page scan of the same document can be 40 MB. The difference is not the number of pages, it is what those pages contain.
Almost every oversized PDF comes down to one of these:
- Scanned pages. Each page is a full resolution photograph, frequently at 600 DPI when 150 would be indistinguishable on screen.
- Photos inserted at camera resolution. A 4000 pixel photo placed in a document that displays it at 500 pixels carries sixty times the data needed.
- Embedded fonts. Usually small, occasionally significant when many font families are embedded in full.
- Invisible leftovers. Cropped image areas, deleted content and revision history that the file still carries.
Knowing which applies tells you whether you can compress a PDF substantially or barely at all.
How to compress a PDF
- Open the compressorDrop your PDF on. Nothing is uploaded, so even a 50 MB file opens straight away.
- Set a target sizeEnter the limit the portal or email requires, in megabytes.
- Check the previewLook at a page with small text before committing. This is the step people skip and then regret.
- DownloadYou get a smaller file, and the original on your disk is untouched.
The important trade-off. Compression works by resampling the images inside the pages. On a scanned document, that means the text, which is part of the image, becomes softer. If your PDF has selectable text, compression may convert pages to images and the text stops being searchable. Always check the preview.
Scan to open
Set the limit in megabytes and see a live preview before you download. Nothing uploaded.
How much smaller can it get?
| Document type | Typical reduction |
|---|---|
| High resolution scan | 70 to 90 percent |
| Report with large photos | 50 to 80 percent |
| Presentation exported to PDF | 40 to 70 percent |
| Plain text document | 5 to 15 percent |
The pattern is simple: compression removes image data. A document that is mostly images shrinks dramatically. A document that is mostly text is already small and has little to give, which is why a 300 KB text PDF stays roughly 300 KB no matter what you do.
Sometimes compression is the wrong tool
Before compressing, check whether one of these fits better.
You only need some of the pages
Extracting five pages from a fifty page document reduces the size by ninety percent with zero quality loss. The split tool does this instantly.
The photos were inserted at full resolution
If you built the PDF yourself, resize the images first and rebuild it. Downscaling a 4000 pixel photo to 1200 before insertion gives a far better result than compressing afterwards, because you control the quality rather than an algorithm.
It is a scan you need as text
Compressing a scan makes the text softer. If what you actually need is the words, run OCR instead and work with the text, as covered in extracting text from an image.
Scanning better in the first place
Most oversized PDFs are created rather than encountered. Two settings prevent the problem entirely.
Resolution: 150 DPI is fine for anything read on screen, 300 DPI is enough for print. Scanning ordinary documents at 600 DPI quadruples the file size for detail nobody will ever see.
Colour mode: a black and white text document scanned in full colour is roughly three times larger than the same page in greyscale. Unless the original has colour that matters, greyscale is the right choice.
Getting these two right at scan time usually removes any need to compress afterwards.
Email limits worth knowing
Most providers cap attachments around 25 MB, and many corporate systems are stricter at 10 MB. If a document sits just above the line, compressing is quicker than uploading it to a file sharing service and sending a link.
If you are attaching several documents, merge them into one file first and compress the result, which is both tidier for the recipient and usually smaller than the parts sent separately.
Scan to open
Extract any page range with no quality loss at all. Often better than compressing.
Frequently asked questions
Why did my PDF barely get smaller?
It is probably mostly text, which is already stored very efficiently. Compression removes image data, so a document without large images has little to give.
Does compressing a PDF reduce quality?
Yes, images are resampled. On a scan that means the text becomes softer, since the text is part of the image. Check the preview on a page with small text before downloading.
Will the text still be searchable afterwards?
Not always. Heavy compression can convert pages to images, which removes the text layer. If searchability matters, compress lightly or use the split tool instead.
What resolution should I scan at?
150 DPI for documents read on screen, 300 DPI if they will be printed. Scanning at 600 DPI quadruples the file size for detail that is never visible.
Is my document uploaded to a server?
No. Compression runs inside your browser, which matters because PDFs are usually statements, contracts and scans of identity documents.
What is the maximum email attachment size?
Most providers allow around 25 MB, though many corporate systems cap at 10 MB. Compressing is usually quicker than switching to a file sharing link.
Compress a PDF online, free and privately
To compress a PDF effectively, it helps to know why yours is large. Scanned pages, photos inserted at camera resolution and fully embedded fonts account for almost every oversized document. Scans compress dramatically, often by 70 to 90 percent, while a plain text PDF is already efficient and will barely shrink at all.
Compression works by resampling the images inside the pages, so there is a genuine trade-off. On a scanned document the text is part of the image and becomes softer, and pages with selectable text may be converted to images, which stops them being searchable. Our compressor shows a live preview for exactly this reason, so you can judge the result before downloading.
Often a different tool fits better. Extracting the pages you need with the split tool reduces size with no quality loss at all, and if what you want from a scan is the words, OCR is the right route. Everything runs in your browser, so statements, contracts and scans are never uploaded, and you can merge or edit the document in the same session.
