Why Compress PDF Files?
Large PDF files are frustrating. They're slow to upload, exceed email attachment limits, and eat up storage space. Compressing PDFs solves these problems while keeping your documents readable.
Common reasons to compress PDFs:
- Email attachments — Most email providers limit attachments to 25MB
- Faster uploads — Smaller files upload quicker to cloud storage and web forms
- Save storage — Reduce the space needed on your device or cloud
- Website performance — Smaller PDFs load faster for website visitors
How to Compress PDF with ModernPDF (Recommended)
The safest way to compress PDFs is with ModernPDF's free compression tool. Your files never leave your browser — all processing happens locally on your device.
- Go to ModernPDF Compress
- Drag your PDF into the upload zone or click to browse
- Select your compression level (low, medium, or high)
- Click "Compress PDF"
- Download your smaller file
Understanding PDF Compression
PDF compression works by optimizing the internal components of a PDF file:
Image compression
Images typically account for 80-90% of PDF file size. Compression reduces image resolution and applies efficient encoding. Modern algorithms like JPEG 2000 can achieve significant size reduction with minimal visible quality loss.
Font optimization
PDFs often embed entire font files even when only a few characters are used. Subsetting fonts removes unused characters, reducing file size.
Object stream compression
Internal PDF structures can be compressed using algorithms like Flate encoding, reducing overhead without affecting content.
Choosing the Right Compression Level
The best compression level depends on your use case:
- Low compression — Use for documents you'll print or where image quality is critical (photos, design work)
- Medium compression — Best for most business documents, reports, and general sharing
- High compression — Use when file size is the priority (email attachments, web downloads)
Tips for Better Compression Results
- Start with the original — Compressing an already-compressed PDF yields poor results
- Consider the content — Text-heavy PDFs compress less than image-heavy ones
- Check the output — Always review compressed files before sharing to ensure quality is acceptable
- Keep the original — Save your uncompressed original in case you need it later
Alternative Methods
Adobe Acrobat Pro
Adobe's "Reduce File Size" and "Optimize PDF" features offer granular control over compression settings. However, this requires a paid subscription ($12.99/month or higher).
macOS Preview
On Mac, you can export a PDF with reduced file size: File → Export → Quartz Filter → Reduce File Size. The results are inconsistent and often reduce quality more than necessary.
Frequently Asked Questions
How much can I reduce PDF file size?
Depending on the content, you can reduce PDF size by 50-90%. PDFs with lots of images compress more than text-heavy documents. A 10MB PDF with photos might compress to 1-2MB, while a 10MB text document might only reach 7-8MB.
Will compression reduce PDF quality?
ModernPDF offers multiple compression levels. Low compression maintains near-original quality with modest size reduction. High compression significantly reduces size but may slightly reduce image sharpness. Text remains crisp at all compression levels.
Is online PDF compression safe?
With ModernPDF, compression happens entirely in your browser using JavaScript. Your files are never uploaded to any server, making it the safest option for confidential documents like contracts, financial statements, or medical records.
Can I compress a PDF multiple times?
You can, but each compression pass yields diminishing returns and may degrade quality. It's better to use a higher compression setting once than to compress multiple times.