Most PDFs are big for one reason: images. A scanned contract is really a stack of photos of paper, and a report exported with full-resolution pictures can easily run to tens of megabytes. The text itself takes up almost no space. That's why the S-PDF compress PDF tool works on the images first, re-encoding them at a sensible resolution while your text stays crisp and selectable.
Choose how strong the compression should be, drop in one or several PDFs, and the result screen shows the size before and after, plus the percentage saved. If a file can't be made any smaller, you simply get your original back unchanged — you will never download a bigger file than the one you uploaded.
How to compress a PDF in 3 steps
- Add your PDFs Drag one or more PDFs onto the upload box or select them from your device. Several files can be compressed in the same task.
- Choose a compression level Recommended suits most documents. Pick Extreme for the smallest possible file, or Less compression when the PDF will be printed. Tick Convert to greyscale to save even more.
- Compress and download Start the compression, check the before → after sizes, then download. Several files arrive as one ZIP, and each can also be downloaded on its own.
Why use S-PDF for Compress PDF
- Three clear levelsExtreme, Recommended and Less compression map to 72, 150 and 300 dpi images, so you know exactly what you are trading.
- Text stays sharpText and vector graphics are never turned into pictures, so they remain crisp, searchable and selectable.
- Never biggerIf a PDF cannot be made smaller, you get the original file back untouched.
- See the savingsThe result screen shows the size before and after and the percentage saved for every file.
- Batch compressionCompress several PDFs at once and download them together as a ZIP or one by one.
- Private by defaultFiles travel over HTTPS and are deleted automatically one hour after your last action.
Which compression level should you choose?
| Level | Images | Best for |
|---|---|---|
| Extreme | 72 dpi | Email attachments, strict upload limits, reading on screen |
| Recommended | 150 dpi | Everyday documents, sharing and normal office printing |
| Less compression | 300 dpi | Files you will print, detailed scans, photos that must stay sharp |
Not sure? Start with Recommended. If the file is still too big, run it again with Extreme. Convert to greyscale is worth ticking for scanned letters, forms and invoices where colour adds nothing, because grey images carry less data than colour ones. Leave it off for charts, photos or anything where colour carries meaning.
What happens to your PDF behind the scenes
Compression runs in two passes. First, Ghostscript re-encodes the images at the resolution of the level you chose, subsets and compresses embedded fonts so only the characters you actually use are kept, and removes duplicate images, so an image embedded several times is stored only once. Then qpdf runs a lossless pass that packs the file's internal objects into object streams and applies maximum Flate compression. That second pass doesn't change how a single page looks.
Text in a digital PDF is stored as characters and shapes, not pixels, so it stays sharp at every level. Only images lose detail. Keep in mind that a scanned page is an image: at Extreme, very small print in a scan can turn soft, so open the result and zoom in before you send it.
When a PDF barely shrinks — and what to do next
Files that were already optimised, contain mostly text, or were compressed before may hardly get smaller. In that case there is little left to remove, and you'll get the original back rather than a worse copy. A few other ways to reduce the size:
- Drop pages you don't need with Remove pages, such as blank scan pages or long appendices.
- Send it in parts. Gmail accepts attachments up to 25 MB; for a larger document, split the PDF into pieces or share a cloud link instead.
- Compress after merging. If you merge PDFs into one file, compress the final result once rather than each piece separately.
- Scan smarter next time. 300 dpi greyscale is plenty for most paperwork and produces far smaller files than a high-resolution colour scan.
Frequently asked questions
Is it free to compress PDF files?
Yes. You can compress up to 10 files and 50 MB per task, 5 times a day, with no signup and no watermark. Pro ($5 a month or $50 a year) allows 50 files and 500 MB per task, unlimited tasks under fair use, and no ads.
Will compressing a PDF reduce its quality?
Images are re-encoded at a lower resolution, so photos and scans lose some fine detail, most noticeably at Extreme. Text and vector graphics are not affected. Less compression keeps images at 300 dpi, which is print quality.
How do I get a PDF under 25 MB for Gmail?
Try Recommended first. If the file is still over 25 MB, run it again with Extreme and Convert to greyscale. If it is still too large, split it into parts or share it as a cloud link.
Why did my PDF hardly get smaller?
It was probably already optimised or contains mostly text, which takes very little space. When a file cannot be made smaller, you receive the original unchanged instead of a larger copy.
Can I compress several PDFs at once?
Yes. Add up to 10 files on the free plan or 50 with Pro. They are compressed with the same settings and delivered as one ZIP, and each file can also be downloaded individually.
Is the text still searchable after compression?
Yes. The text layer is kept, so you can still search, select and copy text. A scan without OCR never had a text layer; use the OCR PDF tool to add one.
Are my files safe?
Files are sent over HTTPS, stored in a private folder that is not reachable from the web, never shared or used for anything else, and deleted automatically one hour after your last action. The Delete my files now button removes them immediately.
Does it work on a phone or a Mac?
Yes. It runs in any modern browser on Windows, Mac, Linux, Chromebook, iPhone, iPad and Android, with nothing to install.
Last updated by The S-PDF team.