Gmail bounced it: 18MB, limit 25MB — technically under, practically blocked because the recipient's server capped at 10MB. The deck had to fly that night. Medium compression took it to 6MB in seconds, text still razor sharp, photos indistinguishable on screen. PDF compression is a level choice, not a button: Light, Medium and Strong trade bytes for fidelity differently per content type. This guide maps each level to email and portal scenarios with a zoom-to-verify protocol.
Part of the PDF workflow guide. Compress in PDF compress; slim oversized sources first via merge planning. Scanned-image limits in text vs OCR.
Light, Medium, Strong: pick by destination
| Level | Saving | Image DPI | Use when |
|---|---|---|---|
| Light | ~20% | Near-original | Archival copies, print handouts |
| Medium | ~50% | ~150 DPI | Email, uploads, portals |
| Strong | ~70% | Aggressive | Phone viewing, strict 1–5MB caps |
My 18MB photo deck: Light → 14MB, Medium → 6MB, Strong → 3.4MB. Text stayed vector-sharp at every level (only images downsample) — which is why born-digital files compress so well and scanned pages (flat images, no text layer) save just 20–30%. My rule: Medium for everything unless a stakeholder names a different constraint.
Verify at 100%: the zoom protocol
- Zoom a photo page to 100% and compare faces, gradients and fine lines against the original.
- Zoom small text (footnotes, captions, axis labels) — the first casualty of Strong.
- Check file size against the destination cap with margin: Gmail 25MB theoretical, 10MB practical for corporate servers; portals per their notice.
- Keep the original archived — recompressing a compressed file compounds artifacts. Always compress from the master.
Locked files and scanned limits
Two hard stops: encrypted PDFs refuse compression until unlocked with the owner password (user passwords merely open viewing — remove restrictions properly), and image-only scans save little because there is no vector text to preserve. A 300-page set over 200 pages should split first (see range extraction), compress parts, then rejoin. On phones, process under-50MB jobs per pass and keep the tab foreground — backgrounded tabs throttle workers.
General guidance only. For print production, never compress the press file — compress a distribution copy and archive the print master separately.