Why a Merged PDF Gets Bigger
Many people assume that merging multiple PDFs into one simply adds up the file sizes. In practice, the merged result is often larger than the sum of the originals, for three reasons: first, the fonts embedded in each PDF are kept separately; second, metadata and cross-reference tables grow; third, some merging tools repackage objects and create redundancy. If the originals contain high-resolution scanned images, the merged file can balloon even more.
Merging is only the first step. To make the file easy to email, send over chat apps, or upload to a website, you often need to compress it again afterward.
The Right Order: Merge Then Compress
The recommended order is: merge first, then compress. If you compress first and then merge, the merge tool may unpack the compressed streams and repackage them, which wipes out the earlier compression.
- Use the PDF Merge Tool to combine multiple files into one in order
- Check that the page count and order of the merged result are correct
- Then use the PDF Compress Tool to compress the merged file a second time
- Download the final file and confirm the size and clarity are acceptable
Key Dimensions of Compression
Image downsampling
Images are usually the biggest part of a PDF. Scanned documents often run 300 DPI or higher, while 150 DPI is enough for screen reading. Compression tools resample images to a target resolution, which can cut the size by more than half. Note: if you plan to print, don't downsample too much—keep it at 200 DPI or above.
Re-encoding images
Converting uncompressed or BMP-encoded images to JPEG (for photos) or PNG (for transparency/line art) pays off immediately. For black-and-white scanned text, the CCITT G4 encoding is the smallest.
Font subsetting
Embed only the glyphs the document actually uses, rather than the entire font. Chinese PDFs benefit the most—a single Song typeface can be over ten megabytes, but after subsetting only a few hundred characters remain.
Cleaning up redundant objects
Merging can leave behind unreferenced objects, duplicate XObjects, and obsolete metadata. Running a clean during compression can save another 5%–15%.
Compression Strategies for Different Cases
| Case | Recommended Strategy | Expected Size |
|---|---|---|
| Email attachment (25MB limit) | Medium compression, images at 150 DPI | 30%–50% of original |
| Chat app sending (100MB) | Light compression, keep 200 DPI | 50%–70% of original |
| Website download | Stronger compression, images at 120 DPI | 20%–40% of original |
| Archiving | Light compression, prioritize clarity | 60%–80% of original |
Common Mistakes
Chasing the smallest size blindly
Compressing too hard makes text blurry and images blocky. For contracts with stamps and signatures, over-compression can even spark disputes over legal validity. Test one copy first, confirm the clarity, then batch process.
Forgetting compression is lossy
Every recompression can lose quality. Keep the original merged file, name the compressed version separately, so you can regenerate it later if needed.
Practical Advice
DocsAll's merge and compress tools both run locally in your browser—files are never uploaded, which makes them suitable for sensitive documents like contracts and reports. Remember the flow: merge first, then compress. Pick different compression levels based on the purpose; don't just use the strongest setting for everything.
When the merged file is still too large, use the compress tool to inspect what's taking up space—if images dominate, downsample them; if fonts do, subset them. Treating the symptom directly beats blindly maxing out compression.