Table of Contents
Why Compress Locally?
The Privacy Problem with Online Compressors
Most online compressors upload your document to a server, process it, and return the result. This means your confidential data leaves your device, files may be stored temporarily on third-party infrastructure, and you're trusting a vendor's security practices.
Our Client-Side Solution
Our free PDF compressor runs 100% client-side using WebAssembly. Your file never leaves your browser's memory. No uploads, no server, no exceptions.
Compression Methods Explained
1. Image Downsampling (Biggest Savings)
PDFs often contain images at 300+ DPI—far higher than needed for screen viewing. Downsampling to 150 DPI (screen) or 72 DPI (web) can reduce file size by 60–80% with minimal visual impact.
2. Image Recompression
Re-encoding JPEG images at lower quality (e.g., 75% instead of 95%) saves significant space. For PNG images, converting to JPEG where transparency isn't needed yields massive reductions.
3. Font Subsetting
Embedded fonts often contain thousands of glyphs. Subsetting keeps only the characters actually used in the document.
4. Metadata & Object Cleanup
Removing duplicate objects, unused resources, and bloated metadata (XMP, thumbnails, form fields) trims overhead without affecting visible content.
Choosing the Right Settings
| Document Type | Recommended Setting | Expected Reduction |
|---|---|---|
| Scanned documents | 150 DPI, JPEG 75% | 70–90% |
| Presentation slides | 150 DPI, JPEG 80% | 50–70% |
| Text-heavy reports | Font subsetting + cleanup | 30–50% |
| Mixed content | Balanced (default) | 40–60% |
| Archival/print | 300 DPI, minimal compression | 10–20% |
Step-by-Step Compression
- Open Compress PDF
- Drag your file into the upload zone
- Select compression level:
- Maximum: Smallest file, some quality loss
- Balanced (default): Best quality/size tradeoff
- Minimal: Preserves quality, modest reduction
- Click Compress — processing takes seconds
- Download the optimized PDF
Before/After Example
A 45 MB scanned contract (300 DPI, color images):
- Maximum: 3.2 MB (93% reduction)
- Balanced: 8.7 MB (81% reduction)
- Minimal: 22 MB (51% reduction)
All versions remain fully readable. The balanced version is indistinguishable from the original on screen.
Privacy First
Zero bytes of your document leave your device. The compression algorithm runs in a WebAssembly worker. No network requests. No server. No exceptions.
When Not to Compress
- Print-ready files for professional printing (keep 300 DPI)
- Legal/archival originals where fidelity is paramount
- Files with fine details (medical imaging, engineering drawings)
Combine with Other Tools
After compressing, you might want to:
- Merge multiple compressed PDFs
- Add page numbers for organization
- Protect with password for security
- Convert to images for web publishing
Try it free: Open the Compress PDF tool and reduce your file sizes in seconds. No account, no uploads, no limits.