PDF files can grow surprisingly large. A single-page scanned document can be 5MB. A presentation exported to PDF can be 50MB. A multi-chapter report with high-resolution images can be hundreds of megabytes. Email attachments get rejected. Upload forms show size errors. Website download pages are slow. PDF compression solves all of these problems — if done correctly.
Why PDFs Get Large
PDF file size is primarily driven by the images embedded within them. A PDF containing only text is typically very small — even a 100-page text document is often under 500KB. The moment you add images, photographs, or scanned pages, the file size increases dramatically.
The three main contributors to PDF file size are:
- Embedded images at full resolution — A Word document exported to PDF embeds images at their original resolution, which may be far higher than necessary for the intended use
- Uncompressed or lightly compressed image data — Some PDF creation tools use minimal compression for images to preserve quality
- Embedded fonts — Fonts embedded in PDFs add file size, though typically less than images
How Ghostscript Compresses PDFs
Ghostscript is the professional open-source PostScript and PDF interpreter used by print shops, publishing houses, and document management systems worldwide. It compresses PDFs by resampling embedded images to appropriate resolutions for the target use case, applying JPEG compression to colour and greyscale images, and applying CCITT compression to black-and-white images.
FlipFiles Pro uses Ghostscript for all PDF compression — the same engine that powers Adobe Acrobat Distiller and many commercial document management systems.
Compression Settings Explained
Ghostscript offers four main output profiles for PDF compression:
| Setting | Image Resolution | File Size | Best For |
|---|---|---|---|
| Screen (/screen) | 72 DPI | Smallest | On-screen viewing only, web download |
| eBook (/ebook) | 150 DPI | Small | Email, general sharing, tablets |
| Printer (/printer) | 300 DPI | Medium | Desktop printing, professional documents |
| Prepress (/prepress) | 300 DPI + colour managed | Larger | Commercial printing, colour-critical work |
Typical Compression Results
Real-world compression ratios depend heavily on the original file content:
- Scanned document (300 DPI original) compressed to ebook: typically 60-75% smaller
- Presentation PDF with photos compressed to screen: typically 70-85% smaller
- Text-heavy report compressed to ebook: typically 20-40% smaller
- Already-compressed PDF: minimal additional reduction possible
PDF Grayscale Conversion
Converting a colour PDF to grayscale achieves significant size reduction — typically an additional 20-40% on top of compression — because colour images require three channels (RGB) while grayscale images require only one. For documents that do not need colour (most text reports, legal documents, financial statements), grayscale conversion is a highly effective size reduction technique. FlipFiles Pro's PDF Grayscale tool uses Ghostscript to convert the colour space cleanly.
What Browser-Based Compression Tools Miss
Most browser-based PDF compression tools use JavaScript-based PDF manipulation libraries. These libraries can remove unnecessary metadata and apply basic compression to image streams, but they do not resample images to lower resolutions — the most effective compression technique. A browser tool that claims to compress your PDF from 10MB to 8MB is not really compressing it. Ghostscript compressing the same file to screen resolution will typically achieve 1-2MB.
Batch Compression
If you have multiple PDFs to compress — a month's worth of supplier invoices, a year of contracts, a library of reports — FlipFiles Pro's Batch PDF Compress tool allows you to upload multiple PDFs simultaneously. All are compressed with your chosen settings and returned as a single ZIP file. This is considerably faster than compressing files one by one.