How to Compress a PDF Without Losing Quality

A 50 MB PDF is almost always a sequence of high-resolution photos wrapped in a PDF container. The PDF format itself adds almost no overhead. The images are doing the damage. Compress the images, the PDF shrinks dramatically.

A typical 30 MB phone-scanned receipt bundle compresses to 2–4 MB with no visible difference. A 100 MB catalogue PDF often compresses to 8–15 MB. The compression is invisible because the original images were stored at print resolution (300 DPI) but will only ever be viewed on screen (where 150 DPI is indistinguishable).

What makes a PDF large

1. Embedded full-resolution photos. Phone scans, product shots, screenshots saved at 4× retina resolution. By far the biggest contributor.
2. Embedded fonts. Each non-system font adds 50–500 KB. Usually not worth removing.
3. Vector graphics and forms. Tiny. Almost never the cause.
4. Metadata, attachments, JavaScript actions. Usually negligible.

If a PDF is huge and contains no images, embedded video is the likely culprit, but most PDF tools (including this one) leave embedded video alone.

Compression presets that match real use cases

• Email-ready (under 10 MB). 150 DPI image downsampling, JPEG quality 80. Indistinguishable on a phone or laptop screen. The right default.
• Web upload (under 5 MB). 120 DPI, quality 75. Still fine for screen viewing, noticeably smaller.
• High quality (preserve printability). 200 DPI, quality 90. Print at A4 with no visible loss. Use when you might print the result.

What compression cannot help with

• PDFs that are already heavily compressed. Running them through again typically shaves 5–10% at best.
• PDFs whose bulk comes from vector content (CAD drawings, complex forms). The images are already small; the geometry is what is large.
• Password-protected PDFs. Most browser-based tools cannot open these without the password.

Compress in your browser

Use the PDF compressor. Drop the PDF in, pick a preset, download. The file never leaves your machine. Compression runs in JavaScript locally. Useful for contracts, financial statements, and anything else you would rather not upload to a third-party site.