You have a 47 MB PDF to email, but Gmail blocks above 25 MB. You compress it on some online site, and the result looks like a photocopy left in the oven. Frustrating.
Good news: compressing a PDF without visible quality loss is technically possible, as long as you understand what's happening under the hood and pick the right tool.
Where does PDF weight come from?
A PDF is a container that can host several content types:
- Vector text: ultra-light (a few KB even for 100 pages)
- Embedded fonts: 50-500 KB per font
- Bitmap images (photos, scans): the #1 cause of heavy PDFs — 100 KB to 5 MB per image
- Metadata and structure: marginal
- Layers, forms, signatures: marginal
99% of excessive weight comes from images. A text PDF of 50 pages rarely exceeds 200 KB. A PDF with 50 pages each containing a 300 DPI photo easily hits 50 MB.
The 4 compression levers
1. Image resolution reduction (downsampling)
An A4 page printed at 300 DPI is sufficient for professional print quality. But on screen, 150 DPI is largely enough — and 96 DPI is invisible to the naked eye for reading.
Reducing 300 DPI → 150 DPI divides image weight by 4 with no visible loss on screen.
2. Image re-encoding (JPEG vs PNG)
PNG scans (lossless) weigh 5 to 10 times more than the same image in high-quality JPEG. Good compressors automatically convert photographic PNGs to JPEG quality 85 — invisible to the eye, divides weight by 5-10.
3. Font subsetting
A PDF with embedded fonts integrates the entire font — including characters never used (Chinese ideograms, Cyrillic alphabets, math glyphs). Subsetting keeps only the actually-used characters.
Typical gain: 30 to 200 KB per font. Marginal but accumulated over a document with 5 fonts, that's 1 MB.
4. Removing unused data
PDFs often contain orphan objects: old page versions, software metadata, preview thumbnails, duplicated fonts. A deep cleanup (-clean in qpdf) gains 5-15% with no loss.
Compression levels: when which?
| Use case | Recommended level | Typical reduction | Quality |
|---|---|---|---|
| Common email attachment | Medium | -60% | Imperceptible |
| Document to archive | Light | -20% | Identical |
| Web or mobile display | High | -85% | Good on screen, mediocre to print |
| Professional print | None or very light | -10% | Print intact |
Many tools offer a single slider. A good tool exposes at least 3 levels: low (quality preserved), medium (compromise), high (web).
Free tools comparison
I tested in 2026 on the same 47 MB PDF (annual report with high-resolution photos).
| Tool | "Medium" level | "High" level | Privacy | Free limit |
|---|---|---|---|---|
| PDFly | 9.2 MB | 4.1 MB | ⭐⭐⭐⭐⭐ Browser-side | 50 MB |
| iLovePDF | 8.8 MB | 3.9 MB | ⭐⭐⭐ US upload | Limited |
| SmallPDF | 9.5 MB | 4.5 MB | ⭐⭐⭐ Swiss upload | 1 file/day |
| Adobe Acrobat | 10.1 MB | 5.2 MB | ⭐⭐⭐ US upload | 2 files/day |
| Ghostscript (CLI) | 8.9 MB | 3.7 MB | ⭐⭐⭐⭐⭐ Local | None |
At equivalent quality, PDFly and iLovePDF are tied on raw performance. The difference is the architecture: PDFly does everything in your browser, the latter sends to US servers.
For Adobe, the result is slightly heavier at comparable quality — paradoxical given the price (€19/month).
How to compress on PDFly
- Go to pdfly.eu/en/tools/compress
- Drop your PDF (up to 50 MB on the free tier)
- Choose the compression level:
- Light: roughly -20%, identical quality
- Medium (recommended): -60%, ideal for email
- High: -85%, ideal for web
- Click "Compress" → automatic download
Everything happens in your browser. Your files are never sent to our servers.
For very large files (>50 MB) — Premium
For PDFs above 50 MB (illustrated reports, massive high-res scans), PDFly Premium accepts up to 500 MB. Server-side compression also enables more advanced optimizations (font re-encoding, repeated-image deduplication, etc.).
Common mistakes to avoid
- Compressing twice — each compression degrades a bit more. Restart from the original.
- Compressing an already-optimized PDF — gaining 2% at the cost of slight quality loss is rarely useful.
- Compressing a scan at "High" level — text zones become blurry. Maximum "Medium" for scans.
- Compressing then signing — the reverse is better: sign first, compress after (signature adds little weight).
- WeTransfer instead of compressing — for 1 file of 60 MB OK, but if you compress to 6 MB, you avoid third-party dependency (and keep the recipient in their email client).
In summary
Compressing a PDF without visible quality loss is a technically mastered operation since 15+ years. The secret isn't the tool — it's picking the right level.
For 95% of emails and shares, the "Medium" level gives -60% weight for 0% visible on-screen loss.
Compress a PDF now — free, in-browser, no signup.