How PDF Compression Works

Overview

Compression is easier to reason about once you know what is actually taking up the space. In most large PDFs it is images, not text.

What makes a PDF large

• Page images: scans and photographs stored at high resolution.
• Embedded fonts: full font files attached to the document.
• Redundant objects: duplicated resources and metadata.
• Page count: long documents multiply everything above.

Two ways to reduce size

PDF JustFit offers two compression modes, and they reduce size in different ways.

• Smart compression reduces size while keeping text, fonts and document structure fully selectable.
• Maximum compression rasterises pages for the smallest possible file.

How the two-stage solver reaches a target

1. You enter the exact target size you need.
2. The solver searches for a compression result that reaches the target.
3. It preserves as much quality as possible while doing so.
4. You compare the original and the result page by page.
5. You export the optimised PDF.

When there is nothing left to compress

If a PDF is already fully optimised, PDF JustFit keeps your original file rather than producing something larger.

Handling very large documents

Very large scanned documents are handled page by page so that memory stays under control — which matters when the device is a phone or a tablet rather than a desktop computer.

Key facts

• Two-stage solver reaches an exact target size
• Smart mode keeps text selectable
• Maximum mode rasterises pages
• Four presets: Light, Balanced, Strong, Extreme
• Large scans handled page by page

Where compression is used

Compression is the first step in most PDF JustFit workflows. See Exact-Size Compression for the feature and How to Compress a PDF to 2 MB for the walkthrough.