Merge PDFs
Upload, drag to reorder, and merge up to 20 PDFs instantly — free, runs in your browser.
Upload PDFs to preview them here
How It Works
Upload your PDFs
Select or drag in up to 20 PDF files at once. Files are processed entirely in your browser.
Drag to reorder
Arrange the files in the exact order you want them to appear in the merged PDF.
Merge and download
Click Merge — your combined PDF is ready in seconds. No quality loss.
Why SynthPDF?
Browser-only — zero uploads
All merging runs in your browser using PDF-lib. Your files never leave your device, making it safe for contracts, medical records, and confidential documents.
Drag to reorder before merging
Arrange pages visually before you combine — no re-uploading after you change your mind.
Zero quality loss
Merging is a structural operation. Pages are copied as-is — no re-rendering, no compression, no quality reduction.
Up to 20 files in one session
Merge up to 20 PDFs at once on the free plan. Pro and above remove the per-session cap entirely.
No sign-up. No watermarks.
The merged PDF is clean. No SynthPDF branding added, no account gate, no nag screens.
Works on all devices
Merge PDFs on Mac, Windows, Linux, iPhone, or Android — any modern browser works.
What Is PDF Merging — and When Do You Need It?
Merging PDFs combines two or more PDF files into a single, continuous document. The result is one file with all pages in the order you choose, fully portable and universally compatible.
The most common reasons to merge PDFs:
- Combining a cover page, main document, and appendix into one submission file
- Assembling a portfolio from separate project PDFs
- Joining scanned pages from a multi-page document scanned one page at a time
- Bundling invoices or receipts into a single monthly expense report
- Merging chapters of a book or report written in separate files
How PDF Merging Works (Technical)
SynthPDF merges entirely in your browser using PDF-lib, a JavaScript library that reads and writes PDF files without sending data to any server. Each uploaded PDF is parsed into its page tree, and the pages are rewritten into a new PDF object in the order you set — preserving all content, fonts, images, and embedded metadata from each source file.
Because this runs client-side, your files never leave your device. This matters for confidential documents — contracts, medical records, legal filings — where uploading to a third-party server is a risk or a compliance issue.
Merge vs. Insert vs. Append — What's the Difference?
- Merge: Combine two or more complete documents, front to back
- Insert: Add pages from one PDF into the middle of another at a specific position
- Append: Add pages to the end of an existing PDF
Our merge tool supports all three patterns — drag thumbnails to reorder pages from different files into any arrangement before merging.
Tips for the Best Merge Results
- Unlock password-protected files first. Use the Unlock PDF tool, then merge.
- Compress large files after merging. If individual PDFs are large, a quick compression pass after merging keeps the final file manageable.
- Check page orientation. If source PDFs have mixed portrait/landscape pages, use Rotate PDF to align them before merging.
- Name your files descriptively before uploading — the filename list is your only visual cue for multi-file merges.
SynthPDF vs. Adobe Acrobat for Merging
Adobe Acrobat Pro can merge PDFs but costs $19.99/month. Our merge tool is free, runs in your browser, and produces identical output for everyday merge operations. The only case where Acrobat has an edge is extremely large files (500+ pages) or PDFs with complex interactive form fields that need to be merged and remain fillable.
SynthPDF vs. Competitors — Merge PDF Feature Comparison
| Feature | SynthPDF | iLovePDF | Smallpdf | Adobe Acrobat |
|---|---|---|---|---|
| Price | Free | Free (limited) | Free (limited) | $19.99/mo |
| Files per merge | 20 (free) | 15 | 5 | Unlimited |
| Browser-only | ✅ | ❌ | ❌ | ❌ |
| No watermarks | ✅ | ✅ | ❌ (free tier) | ✅ |
| No sign-up needed | ✅ | ✅ | ❌ | ✅ |
| File size limit (free) | 25 MB | 15 MB | 5 MB | None |
Frequently Asked Questions
Free users can merge up to 20 files per session. Pro and above have no per-session limit.
No. Merging is a structural operation — pages are combined as-is with no re-rendering or quality loss.
You need to unlock them first using our Unlock PDF tool, then merge.
Not in merge — use our Split PDF tool first to extract specific pages, then merge those extracted files.
Merge runs entirely in your browser using PDF-lib — your files never leave your device.
Related Tools
Want to learn more?
Read: How to Merge PDFs — Complete Guide →