Why PDFs Get So Large
A PDF with high-resolution images can easily reach 50-25 MB. The main culprits:
- High-DPI images — 300+ DPI images for print embedded in a screen-only document.
- Uncompressed images — some applications export PNG-quality images inside PDFs.
- Full font embedding — all font characters included, not just the ones used.
- Multiple saved versions — PDFs track all changes internally.
- Redundant metadata — thumbnails, comments, hidden objects.
Method 1: KaijuConverter (Online, Free)
No installation, works from any browser:
- Go to Convert PDF to PDF to re-compress.
- Upload your PDF (up to 25 MB free).
- Download the optimized PDF.
Best for quick compression of image-heavy PDFs.
Method 2: Ghostscript (Command Line, Most Powerful)
Ghostscript is the engine powering most PDF compressors online. Using it directly gives you maximum control:
Install:
- Windows: ghostscript.com
- Mac:
brew install ghostscript - Linux:
sudo apt install ghostscript
Commands:
# Screen quality — 72 DPI, smallest file
gs -dPDFSETTINGS=/screen -dNOPAUSE -dBATCH \
-sDEVICE=pdfwrite -sOutputFile=output.pdf input.pdf
# eBook quality — 150 DPI, best balance ← recommended for most use cases
gs -dPDFSETTINGS=/ebook -dNOPAUSE -dBATCH \
-sDEVICE=pdfwrite -sOutputFile=output.pdf input.pdf
# Printer quality — 300 DPI, for printing
gs -dPDFSETTINGS=/printer -dNOPAUSE -dBATCH \
-sDEVICE=pdfwrite -sOutputFile=output.pdf input.pdf
Ghostscript quality levels:
| Setting | Image DPI | Best for |
|---|---|---|
/screen | 72 | Smallest file, screen only |
/ebook | 150 | Email, web sharing (start here) |
/printer | 300 | Quality printing |
/prepress | 300+ | Professional printing |
/default | Varies | Balanced |
Method 3: macOS Preview (Built-in, No Install)
macOS includes a hidden compression option in Preview:
- Open the PDF in Preview.
- File → Export.
- In the Quartz Filter dropdown, select Reduce File Size.
- Save.
Warning: the default "Reduce File Size" filter is aggressive and can reduce image quality significantly. For better control, use Ghostscript.
Method 4: LibreOffice Draw (Free Desktop App)
- Open the PDF in LibreOffice Draw.
- File → Export as PDF.
- In the Images tab, set JPEG compression to 70-80% and reduce DPI.
- Export.
Method 5: Microsoft Word (For Office Users)
If you have Word with a large PDF:
- File → Open the PDF in Word.
- Word converts it to a document.
- File → Save As → PDF, then click Options.
- Under Image Quality, select a lower DPI (96 DPI for screen, 220 DPI for print).
Expected Compression Results
| PDF Content | Method | Typical Reduction |
|---|---|---|
| Text only | Ghostscript /ebook | 20-40% |
| Text + photos | Ghostscript /ebook | 50-70% |
| All photos (catalog, magazine) | Ghostscript /screen | 60-80% |
| Already compressed PDF | Any method | <10% |
| Scanned document | Ghostscript /ebook | 40-60% |
When Compression Doesn't Help Much
If a PDF is already well-compressed, running it through another compressor won't help — and may actually increase the file size as the tool tries to re-process already-optimized data.
Signs a PDF is already compressed:
- File size seems reasonable for the content (under 1 MB per page with images).
- Running through compression gives <5% reduction.
For the Smallest Possible Email-Safe PDF
Target: under 5 MB for attachments. Strategy:
- Use Ghostscript
/screenfor maximum reduction. - Or re-export from the source application at 96 DPI.
- As a last resort, consider sharing via Google Drive or WeTransfer instead.
Related Conversions
- PDF to JPG — extract pages as images
- JPG to PDF — rebuild a PDF from images
- PDF to DOCX — convert to editable format
Related conversions
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