PDFs grow fast. A scanned 30-page contract can balloon to 80 MB. A photo-heavy presentation runs into mail attachment limits. A textbook export is too big for a phone. Compression solves all three — by re-encoding the embedded images at a sane DPI and re-saving the PDF without the bloat.
Zebra runs the entire compression in your browser using WebAssembly. We never see your file. There's no upload, no queue, no third-party server. The same code that runs on a Mac runs on an iPhone — you just drop the PDF in.
Three steps
- Open the PDF compressor.
- Drag your PDF into the dropzone (or click to pick one).
- Choose Light, Medium, or Strong, click Compress, then Download.
That's it. Most 10–30 MB PDFs finish in well under 30 seconds on a modern laptop, and 20–60 seconds on a phone. If you also need to shrink the images themselves before they go into a PDF, check the guide to compressing JPG, PNG and WebP.
Why PDFs get big
Three things usually make a PDF heavy:
- Embedded raster images at print DPI. A single 300-DPI A4 scan is roughly 2480×3508 pixels. Multiply by 30 pages and you're at 80 MB — most of it pixels nobody can see on a phone.
- Uncompressed or weakly compressed image streams. Some PDF creators store images as raw or with low-quality DEFLATE compression. Re-encoding them as JPEG at 85–98% quality typically halves the page size with no visible loss.
- Embedded font subsets bundled multiple times. A common Word-to-PDF bug. Less impact than images but real.
Zebra targets the first two. We re-render each page through pdf.js, re-encode the images at the DPI/quality you chose, and re-bundle the file via pdf-lib. Text stays as text — searchable, selectable, copy-pasteable.
Light, Medium, Strong — what each preset does
Three knobs control the trade-off: render DPI, JPEG quality, and a maximum dimension cap. Here's what each preset uses:
| Preset | Render DPI | JPEG Q | Max side | Typical savings |
|---|---|---|---|---|
| Light | 280 | 98% | 6000 px | 10–30% |
| Medium | 210 | 93% | 4200 px | 40–60% |
| Strong | 170 | 85% | 3200 px | 60–80% |
Light is visually lossless. Use it for documents you'll print, archive, or send to a client who'll zoom in.
Medium is the default. The output is still print-grade — 210 DPI is above the 200-DPI threshold most home and office printers use. Pick this unless you have a specific reason not to.
Strong is for email, web sharing, and mobile. 170 DPI is sharp on any screen and crisp enough for low-stakes print. This is where you'll see the dramatic 60–80% drops.
Why nothing leaves your device
Browser-side PDF compression is unusual. Most free tools — Smallpdf, iLovePDF, PDF24, Adobe's online compressor — upload your file to their server, run the compression there, and email or stream the result back. That model has three problems:
- Your PDF lands on someone else's disk. Even with a "we delete after an hour" promise, there's a window where a misconfigured S3 bucket, an internal employee, or a subpoena can reach it. Contracts, IDs, tax forms, medical records — all the things you'd most want to compress are also the things you least want on a third-party server.
- You upload twice the bandwidth. Once up, once down. On a hotel Wi-Fi or a mobile hotspot, that's the slow part — not the compression.
- Server-side limits force you into a sign-up funnel. "5 MB for free, sign up for 25 MB, pay for 100 MB" is the standard pattern.
Zebra runs everything locally. The file stays in browser memory. There's no upload step, no progress bar for the network, and no way for us to retain or even see what you compressed. If you turn off Wi-Fi after loading the page, compression still works.
When NOT to compress
Compression is lossy on the image streams (text is preserved). A few cases where you should skip it or pick Light. If your PDF is large because of one or two high-resolution images, sometimes it's faster to crop them down to size before re-bundling — then compression on the leaner PDF wins twice.
- PDF/A archival files. Long-term archive PDFs are meant to be byte-stable. Don't compress them.
- Already-optimized small PDFs. If your file is under 1 MB and mostly text, there's nothing to gain. Zebra will detect this and show "0% change" instead of pretending to compress.
- High-detail technical drawings. CAD exports, dense schematics, microscope images — pick Light, or compress the source images separately before exporting.
- Master files for print. Keep the master at full DPI. Compress a copy for email.
Zebra vs Smallpdf vs iLovePDF
A practical comparison, as of May 2026:
| Feature | Zebra | Smallpdf | iLovePDF |
|---|---|---|---|
| Runs in browser | Yes (WASM) | No (server) | No (server) |
| File uploaded | Never | Yes | Yes |
| File size limit (free) | 150 MB | 5 MB | 15 MB |
| Daily limit (free) | 2 / day (anon), 3 / day (signed in) | 2 / day | Limited |
| Watermark | No | No | No |
| Signup required | No | No (small files) | No (small files) |
| Works offline | Yes (after page load) | No | No |
| Three quality presets | Yes | Limited | Yes |
The honest summary: if your PDF is over 5 MB, doesn't fit in iLovePDF's 15 MB free cap, or contains anything private, Zebra is the cleaner path. For tiny files under 5 MB you can use any of the three.
Troubleshooting
"Compression failed"
Usually one of: a password-protected PDF (decrypt it first in Preview on Mac or Adobe Reader), a corrupted PDF that pdf.js can't parse, or a browser running out of memory on a huge file. Try Strong instead of Medium — it caps the maximum side length, which is gentler on memory.
The result is barely smaller
Three possibilities: the PDF is already optimized (text-only or pre-compressed images), the images are tiny vector logos that don't benefit from re-encoding, or you picked Light on a file that needed Strong. Try Strong and compare.
The text looks fine but images are blurry
That's expected on Strong with photo-heavy PDFs. Switch to Medium — at 210 DPI / 93% quality, photos stay sharp on screen and even print acceptably.
It's slow on my phone
WebAssembly on mobile Safari is roughly 3–5× slower than on a Mac. A 30 MB PDF that takes 8 seconds on a M-series Mac may take 30–40 seconds on an iPhone. Plug into power; the iPhone throttles CPU on low battery. If it really hangs, switch to a desktop or use the iOS Files app's built-in "Reduce File Size" Action as a fallback (lower quality, but it works on the device).