justpdf-compress-wasm

PDF compression WASM module for the justpdf project.

Compress PDFs entirely in the browser — no server required, no data leaves the client.

Features

• 4 presets — low, medium, high, extreme
• Image optimization — JPEG re-encoding, downscaling, grayscale conversion
• Font subsetting — keep only used glyphs
• Stream optimization — Flate re-compression, duplicate dedup
• Structure cleanup — unused resource removal, metadata stripping, GC
• Object Stream compression — PDF 1.5+ optimization
• DPI-aware downscaling — CTM-based effective DPI calculation
• Detailed stats — images processed, fonts subsetted, bytes saved, etc.

Build

cargo install wasm-pack
wasm-pack build --target web

Usage (JavaScript)

import init, { compress, compress_advanced, analyze } from './pkg/justpdf_compress_wasm.js';

await init();

const response = await fetch('document.pdf');
const bytes = new Uint8Array(await response.arrayBuffer());

// Analyze without compressing
const info = analyze(bytes);
console.log(`Pages: ${info.pages}, Images: ${info.images}`);

// Compress with preset
const result = compress(bytes, 'high');
console.log(`${result.original_size} → ${result.compressed_size} (${(result.ratio * 100).toFixed(1)}%)`);

// Download compressed PDF
const blob = new Blob([result.data()], { type: 'application/pdf' });
const url = URL.createObjectURL(blob);

// Compress with full control
const advanced = compress_advanced(
  bytes,
  65,     // jpeg_quality (0 = skip)
  150.0,  // max_dpi (0 = skip)
  true,   // font_subsetting
  true,   // remove_unused_resources
  true,   // strip_metadata
  false,  // strip_extras
  false,  // grayscale
);

API

compress(data, preset)

Compress with a preset ("low", "medium", "high", "extreme").

compress_custom(data, jpeg_quality, max_dpi)

Compress with custom JPEG quality and DPI settings.

compress_advanced(data, jpeg_quality, max_dpi, font_subsetting, remove_unused_resources, strip_metadata, strip_extras, grayscale)

Full control over all compression options.

analyze(data)

Analyze a PDF without compressing. Returns page count, image count, total image bytes, and encryption status.

CompressResult

| Property | Description | |---|---| | data() | Compressed PDF bytes | | original_size | Original file size | | compressed_size | Compressed file size | | ratio | Compression ratio (0.0~1.0, lower = more compression) | | images_found | Number of images found | | images_recompressed | Images re-encoded | | images_downscaled | Images downscaled | | images_skipped | Images skipped | | images_grayscaled | Images converted to grayscale | | duplicates_removed | Duplicate streams removed | | objects_removed_gc | Objects removed by GC | | streams_recompressed | Streams re-compressed | | fonts_subsetted | Fonts subsetted | | unused_resources_removed | Unused resources removed | | metadata_items_stripped | Metadata items stripped |

Repository

https://github.com/kihyun1998/justpdf

License

MIT OR Apache-2.0