gs -dPDFSETTINGS=/ebook -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=compressed.pdf merged.pdf). On macOS, use Preview's Reduce File Size filter. On Windows, PDF24 Creator has a built-in compressor. All three work locally with no upload required.
Merging PDFs does not reduce file size — it combines the bytes of all source files into one. If you merge 10 scanned documents at 5MB each, the result is approximately 50MB. For email, web sharing, or upload portals with size limits, that is often too large.
Compression is a separate step. This guide covers every free method to compress PDFs locally — no upload to third-party servers required.
Merge First, Then Compress
PDF Merge & Split handles the merge step — free, local, private.
Add to Chrome — FreeWhy Merged PDFs Are Large
The file size of a PDF depends on its content:
- Scanned documents: Each page is a raster image at 150–600 DPI. High-resolution scans are large — a 300 DPI A4 scan is about 1–2MB per page.
- Photo-heavy PDFs: Embedded photos at full resolution add significant size.
- Text-only PDFs: Very small — typically 50–200KB per page. Text compresses very well.
- Mixed content: Size depends on how many images and their resolution.
Compression primarily works by downsampling embedded images to a lower resolution. Text and vector graphics are not affected — they stay sharp at any size.
Ghostscript Compression Settings
Ghostscript's -dPDFSETTINGS flag provides four preset compression levels:
| Setting | Image DPI | Use for | Typical size reduction |
|---|---|---|---|
/screen |
72 DPI | Web viewing only | 70-80% |
/ebook |
150 DPI | Email, digital distribution | 50-70% |
/printer |
300 DPI | Desktop printing | 20-40% |
/prepress |
300 DPI | Professional print | 10-20% |
Compress with Ghostscript (all platforms)
# For email and digital distribution (recommended for most cases)
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite \
-dPDFSETTINGS=/ebook \
-sOutputFile=merged-compressed.pdf merged.pdf
# For smallest possible file (web/low quality)
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite \
-dPDFSETTINGS=/screen \
-sOutputFile=merged-small.pdf merged.pdfOn Windows with Ghostscript installed, replace gs with gswin64c.
macOS Preview Compression (No Install)
Using the Quartz filter in Preview
- Open your merged PDF in Preview.
- Go to File → Export as PDF.
- Click the Quartz Filter dropdown.
- Select Reduce File Size.
- Click Save.
/ebook settings for more control.
PDF24 Creator (Windows, Free)
PDF24 Creator is a free Windows application with a compression tool that offers a quality slider:
- Download and install PDF24 Creator (desktop version — not the web tool).
- Open the PDF24 application.
- Select "Compress PDF" from the tool list.
- Drag in your merged PDF.
- Adjust the quality slider.
- Click Compress and save the output.
Adobe Acrobat Pro (Paid, Most Control)
Adobe Acrobat Pro's PDF Optimizer gives per-image and per-feature control:
- Set different compression for color images, grayscale, and monochrome (black/white)
- Remove embedded thumbnails, form submission data, and other metadata
- Flatten form fields and layers
For occasional use, Ghostscript or macOS Preview is sufficient. For high-volume or quality-critical workflows, Acrobat's fine-grained controls are worth the cost.
When Compression Helps vs When It Doesn't
Checking Compression Results
After compressing, verify:
- Open the compressed PDF — check that text is still sharp and readable
- Zoom in on images to verify acceptable quality for your use case
- Check the file size (right-click → Properties/Get Info)
- If quality is insufficient, try
/printersettings instead of/ebook
Start With the Merge
PDF Merge & Split handles the merge step free and locally. Compress with Ghostscript afterward.
Install PDF Merge & SplitRelated Guides
- How to Merge PDF Files for Free
- How to Split a Large PDF for Email
- PDF Tools That Work Without Uploading
Frequently Asked Questions
Does merging PDFs increase file size?
Yes — a merged PDF is approximately the sum of its source files' sizes. Merging does not compress content. To reduce the merged file size, run a separate compression step using Ghostscript, macOS Preview, or PDF24.
What is the best free tool to compress a PDF without uploading it?
Ghostscript is the best free local PDF compressor. On macOS, Preview with the Reduce File Size filter also works without installing anything. On Windows, PDF24 Creator has a built-in compressor with a quality slider.
How much can PDF compression reduce file size?
Image-heavy PDFs can be compressed by 60-80%. Text-only PDFs typically compress by only 5-20% since text already compresses well.
Does compressing a PDF reduce text quality?
No — PDF text is stored as vector data, not as pixels. Compression only affects raster images. Text remains perfectly sharp at any zoom level regardless of compression settings applied to images.
What Ghostscript settings should I use for email vs print?
For email: -dPDFSETTINGS=/ebook (150 DPI). For web: /screen (72 DPI, smallest). For print: /printer (300 DPI). For archival: /prepress (maximum quality).
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