Most PDF size advice is a single slider and a shrug. Here is what is actually inside the file, and which parts you can cut before anyone notices.
Why is my PDF so large in the first place?
Open a 40 MB PDF and the instinct is to blame the page count. It is almost never the page count. A page of pure text — fonts, glyph positions, vector rules — costs a few kilobytes. A single full-page photograph scanned at 600 DPI costs several megabytes on its own.
So the size question is really an image question. Three things drive it, in order of impact:
- Resolution. A scanner set to 600 DPI produces four times the pixel data of 300 DPI, and sixteen times that of 150 DPI. Most scanners default higher than anyone needs.
- Encoding. Images stored as PNG or as uncompressed bitmaps inside the PDF can be an order of magnitude larger than the same image as JPEG. Photographs in particular have nothing to gain from PNG.
- Duplicated resources. A merged document can carry the same logo image five times and three copies of the same font subset.
This is why compression results vary so wildly between documents. A file that is 95% photographs shrinks dramatically. A 200-page text-only report is already near its floor, and no tool will do much for it.
What resolution do you actually need?
Resolution is the single biggest lever, and the right value depends entirely on where the document ends up. Guessing high "to be safe" is what produces the 40 MB email attachment.
| Destination | DPI | Effect on size |
|---|---|---|
| Read on a phone or laptop | 110–150 | Baseline — smallest sensible |
| Shared for review, may be printed casually | 150–200 | Roughly 1.8× the 150 DPI file |
| Office printing, contracts, forms | 200–300 | Roughly 4× the 150 DPI file |
| Commercial print, artwork, photography | 300–600 | Do not compress; send the original |
The useful thing buried in that table: dropping from 300 DPI to 150 DPI removes about three quarters of the image data, and on a screen the two are indistinguishable. If the document is going to be read rather than printed, that one change usually does the entire job.
How much does JPEG quality really cost?
After resolution comes the JPEG quality setting — the 0 to 1 slider, sometimes shown as 0 to 100. It behaves less intuitively than people expect, because the relationship between quality and file size is steeply non-linear.
Going from quality 1.0 to 0.8 typically removes half the file size and is essentially invisible. Going from 0.8 to 0.6 removes maybe another third and starts to show faint ringing around sharp edges — most noticeable on text inside a scanned image, least noticeable on photographs. Below 0.5, blocking artifacts appear in flat areas like skies and paper margins.
The practical range is 0.65 to 0.85. Below that you are trading visible damage for diminishing returns; above it you are paying for bytes nobody can see.
One caveat worth knowing: JPEG compression is lossy and it compounds. Compressing an already-compressed PDF a second time damages it more than a single pass at the lower setting would have. If you overshoot, go back to the original file and start again rather than re-compressing the output.
How do you hit a specific size limit like 10 MB or 5 MB?
Most upload limits are hard: 25 MB for Gmail attachments, 10 MB for many university and government portals, 5 MB for a lot of job application forms, 2 MB for some visa systems. Guessing your way down to them wastes time.
A more reliable method, three attempts at worst:
- Start at 150 DPI, quality 0.75. For a scanned document this usually lands between 15% and 30% of the original.
- If it is still over, drop DPI before you drop quality. 120 DPI at quality 0.75 looks considerably better than 150 DPI at quality 0.4 for the same file size.
- If it is still over and the document is long, split it. A 300-page manual will not fit under 5 MB in any readable form — send it as three files, or link to it.
- If it comes in comfortably under, go back up one step. There is no prize for sending a 900 KB file to a 10 MB limit, and the extra quality is free.
The Compress PDF tool shows the resulting size before you download, so those steps happen in the browser without a round trip. And because it runs on your device, there is no limit on the input — you can feed it a 400 MB scan that no upload-based service would accept.
When should you not compress a PDF?
Compression is lossy, and there are documents where that is the wrong trade entirely.
- Anything going to a commercial printer. They want the 300+ DPI original. A compressed file will show artifacts at print size that were invisible on screen.
- Digitally signed documents. Re-encoding the file breaks the cryptographic signature, because the signature covers the exact bytes. If you need it smaller, compress first and sign afterwards with the Sign PDF tool.
- Legal or archival scans where fine detail is the evidence — handwriting, stamps, faint annotations. Keep the original; compress a copy for circulation.
- Text-only PDFs. Already small. If a 12-page text document is 30 MB, something else is wrong — usually a full-page background image embedded on every page.
Is it safe to compress a PDF online?
It depends entirely on the architecture, and most sites do not tell you which one they use.
The conventional approach uploads your file to a server, processes it there, and sends the result back. That works, but your document now exists on someone else’s hardware for as long as their retention policy says — and for a contract, a passport scan or a medical record that is a real consideration rather than a theoretical one.
The alternative is to do the work in the browser. Modern browsers can read local files, decode and re-encode images on a canvas, and rewrite PDF structure through WebAssembly, all without a network request. Every tool on this site is built that way, which is why there is no size cap and no queue: there is no server to be busy.
A quick way to check any tool, including this one: open your browser’s developer tools, switch to the Network tab, and run it. If your file were being uploaded you would see a large outgoing request. There is a longer version of this check in Are online PDF tools safe?
Frequently asked questions
Does compressing a PDF reduce the text quality?
No. Text in a PDF is stored as font glyphs and coordinates, not as pixels, and compression tools leave that untouched — it stays perfectly sharp at any zoom. The exception is a scanned document, where the "text" is really part of a photograph of a page. There, compression does affect it, which is why 150 DPI is the sensible floor for scans.
How much smaller can I expect a PDF to get?
For a scanned document, typically 60–85% smaller at 150 DPI and quality 0.75. For a PDF exported from Word that is mostly text, often under 10% — it was already near its floor. For a slide deck full of photographs, 70–90% is common. The saving is roughly proportional to how much of the file was images.
Can I compress a password-protected PDF?
Not while it is encrypted. The tool cannot read the page contents to re-encode them without the password. Remove the protection first in a PDF reader using the password, compress, then re-apply protection if you still need it.
Will compressing break a digital signature?
Yes. A digital signature covers the exact bytes of the file, so any modification — including compression — invalidates it. Always compress before signing, never after. A visible signature image added as an annotation is not affected the same way, but a cryptographic signature is.
Why did my PDF get bigger after compressing?
This happens when the source images were already more efficiently compressed than the output settings, most often when the PDF contained PNG line art that was re-encoded as JPEG. It is also possible if you set a DPI higher than the original resolution, which upscales rather than shrinks. If the output is larger, keep the original.
Is there a file size limit?
Not on this site. Because compression happens on your own device, the only ceiling is available memory — in practice several hundred megabytes on a typical laptop. Upload-based services cap their free tiers between 5 MB and 100 MB because bandwidth costs them money.