Almost everyone arrives at this problem the same way: something has rejected a file for being too large, and there is a number to get under. The fastest route there depends entirely on what kind of file it is, and the single most effective lever is usually not the one labelled “compress”.
For an image, resize before you compress
This is the one that surprises people. A photo straight off a modern phone is around 4000 pixels wide. If it is going into a document, an email or a web page, it will be displayed at perhaps 1200 — and the file carries roughly eleven times more pixels than anything will ever show.
Halving both dimensions quarters the pixel count, and the file size falls by roughly the same proportion. Nothing visible is lost, because those pixels were never being displayed. Compression, by contrast, buys you a smaller file by degrading the pixels you are looking at.
So the order is:
- Resize to the largest size it will actually be shown at. This is free quality.
- Compress what is left. Around 80% quality is the usual sweet spot — visually indistinguishable from the original for most photographs, and often less than half the size.
- Change format if it is still too big. A photograph saved as PNG will shrink dramatically as a JPG. See choosing an image format for which to pick.
Rough guide to quality settings
- 90–100% — hard to distinguish from the original, and barely smaller. Rarely worth it.
- 75–85% — the useful range. Most people cannot tell at normal viewing size.
- 60–75% — visible softening in detailed areas, acceptable for thumbnails.
- Below 60% — obvious artefacts around edges and text.
For a PDF, find out what is making it big
A PDF is a container, and its size is almost always dominated by one of three things. Which one decides what to do:
- Scanned pages. Each page is a full-resolution image, often 300 DPI or more. This is the usual cause of a 40MB document and the one that compresses best.
- Embedded photographs. Same problem as any image, just wrapped in a document.
- Embedded fonts. A few hundred kilobytes at most — irrelevant unless the document is otherwise tiny.
A text-only PDF exported from a word processor is already small, and compressing it will do close to nothing. If your 30-page report is 200KB, there is nothing to win.
Compressing a PDF works on the images inside it. If that is still not enough, the honest next step is to split it — sending three files of 4MB succeeds where one of 12MB does not, and neither loses anything.
Common limits worth knowing
| Where | Limit |
|---|---|
| Gmail attachment | 25 MB |
| Outlook attachment | 20 MB |
| WhatsApp document | 2 GB |
| Most job application forms | 2–5 MB |
| Government upload forms | Often 2 MB |
Note that email limits apply to the encoded attachment, which is about a third larger than the file on disk. A 24MB file will not go through a 25MB limit.
What not to do
- Don’t zip a JPG or a PDF. Both are already compressed. Zipping typically saves one or two percent and adds a step for whoever receives it.
- Don’t compress the same image twice. Each pass discards more detail. If the result is too big, go back to the original and use a lower quality once, rather than compressing the compressed version.
- Don’t screenshot a document to shrink it. You turn selectable text into an image, which is larger, unsearchable and unreadable to a screen reader.
- Don’t resize up. Enlarging cannot add detail; it only adds pixels and file size.
A note on privacy
Most sites that compress a file for you upload it to a server to do it. For a holiday photo that may not matter. For a signed contract, a passport scan or a medical letter — which is a great deal of what gets compressed to fit an upload form — it very much does.
Every tool linked here runs in your browser: the file is read from disk by the page, processed on your own device, and written back out without ever being transmitted. You can watch the network tab and confirm it.