Why Are PDFs So Large?
PDF files are large when they contain high-resolution images, embedded fonts, or were exported from design software without compression. A PDF from a scanner can easily be 10–50 MB because each page is stored as a full-resolution image (often 300 DPI). A PDF from a Word document is usually smaller because text is stored as vector data, not bitmaps.
How PDF Compression Works
PDF compression primarily works by resampling (downscaling) embedded images to a lower DPI and applying JPEG compression to photos inside the PDF. Text and vector graphics are not affected — they remain sharp at any zoom level. The most aggressive compression setting reduces images to 72–96 DPI (screen resolution), which is fine for on-screen reading but not for printing.
How to Compress a PDF Online
- Go to the PDF Compressor (PDF → PDF conversion with compression).
- Upload your PDF.
- Click Convert to apply standard compression.
- Download the compressed PDF.
Alternatively, convert the PDF to images and then back — though this changes the document structure. For most use cases, the direct PDF compression route is better.
Expected Compression Ratios
| Source PDF type | Typical reduction |
|---|---|
| Scanned document (photos of pages) | 60–80% |
| PDF exported from Word/LibreOffice | 5–20% |
| PDF from design software (InDesign, Illustrator) | 40–70% |
| Already-compressed PDF | 2–5% |
Tips for Maximum Compression
- Remove unused pages before compressing — each page adds to the size.
- Flatten forms and annotations — interactive elements add overhead.
- Use grayscale if the document does not need color — grayscale PDFs are 30–50% smaller.
- Convert charts and diagrams to vector (SVG) in the source document before PDF export — they compress better than raster screenshots.