Merging PDF files used to mean buying expensive software or emailing sensitive documents to a thirdparty server. Today, you can combine any number of PDFs directly in your browser for free, in seconds, with your files never leaving your device.
This guide covers everything = how to use PDF Size Reducer's free merge tool, what actually happens under the hood, how to reorder pages and answers to the most common questions.
How to Merge PDF Files- Step-by-Step
The whole process takes under a minute. Here's exactly what to do:
Open the Merge PDF tool
Navigate to PDF Size Reducer > Merge PDF. You'll see a drop zone - no sign-up prompt, no ad interstitials.
Upload your PDF files
Drag and drop multiple files at once onto the drop zone, or click "Select PDF Files" to use the file picker. You can add up to 20 files (500 MB combined). Files load into a sortable list - you'll see the name and page count of each.
Reorder files if needed
Drag the file cards to rearrange them. The final PDF will follow this order exactly from top to bottom. You can also remove individual files using the trash icon.
Click "Merge PDFs"
Hit the green button. Your browser processes all the files using the pdf-lib JavaScript library. For most files, it completes in 1-3 seconds.
Download your merged PDF
The result panel shows a file card with your merged PDF's name and page count. Click "Download Merged PDF". Done. The file saves directly to your Downloads folder.
Ready to try it?
Merge your PDFs right now — free, instant, and completely private.
Open Merge PDF ToolIs It Safe to Merge PDFs Online?
This is the most common concern and reasonable one. When you use a server-based PDF tool (most of them), your files travel across the internet to a company's servers, get processed, and are (hopefully) deleted. You're trusting their security practices and data-retention policies.
PDF Size Reducer works differently. It's 100% browser-based= your PDF files are read into memory in your browser tab, processed by a JavaScript library, and a new merged file is generated - all without any network request containing your file data. You can verify this yourself by opening DevTools > Network tab and watching while you merge.
ArrayBuffer objects in your tab's memory - the only thing
that ever leaves is the final file download, which goes directly to your disk.
Browser based vs. Server-based= Quick Comparison
| Feature | PDF Size Reducer (Browser) | Server-based tools |
|---|---|---|
| Files uploaded to server | Never ✓ | Always ✗ |
| Works offline (after page load) | Yes ✓ | No ✗ |
| Processing speed | Instant (local CPU) | Depends on server load |
| File size limits | 500 MB combined | Varies (often 25–100 MB on free tier) |
| Account required | No ✓ | Often yes ✗ |
| Ads or upsells | None ✓ | Frequent ✗ |
What Happens When You Merge PDFs?
Understanding the mechanics helps you troubleshoot edge cases. Here's what pdf-lib does when it merges your files:
Page copying, not re-encoding
pdf-lib copies page objects - it reads each page's content stream, resources (fonts, images, colour spaces), and metadata, then writes them into a new PDF document. This means:
Image quality is perfectly preserved. No re-encoding, no JPEG recompression, no quality loss. A 300 DPI image stays at 300 DPI after merging. The merged file will typically be within 1–5% of the combined size of the original files.
What can go wrong
Occasionally a merge will fail or produce a broken file. Here are the most common causes:
Tips for Better PDF Merging
Consistent page sizes
If you're merging an A4 document with a US Letter document, the result will have mixed page sizes - which can look odd in some PDF readers. For a clean result, ensure all source files use the same paper size before merging.
Compress before merging large files
If your combined PDFs are very large and you want a smaller output, consider optimizing each PDF first. Browser-based optimization can remove 10–40% of structural bloat from Word-exported PDFs. Then merge the lighter files.
Split first, then merge strategically
Sometimes you only want certain pages from a document. Use PDF Size Reducer's Split PDF tool to extract just the pages you need, then merge those extracts into your final document.
File naming after merge
The tool names your output merged.pdf by default. Rename it to something
meaningful before archiving a good pattern is YYYY-MM-DD_project_final.pdf.
Alternative Methods to Merge PDFs
PDF Size Reducer is the fastest option for most people, but here are the other common approaches:
macOS - Preview app (built in)
Open a PDF in Preview > View > Thumbnails > drag other PDFs' thumbnails into the sidebar. File > Export as PDF when done. Works without any extra software.
Windows — Microsoft Print to PDF
Open each PDF in Edge browser > Print > "Microsoft Print to PDF". For true merging (not screenshot-quality) combine via Adobe Acrobat or a browser tool like PDF Size Reducer.
Command line — Ghostscript
For developers or power users who need to merge in batch scripts =
gs -dBATCH -dNOPAUSE -sDEVICE=pdfwrite \
-sOutputFile=merged.pdf \
file1.pdf file2.pdf file3.pdfGhostscript is free, open-source, and available on Mac (via Homebrew), Linux (via apt), and Windows. It's the go-to for automation pipelines.
Python - pypdf
from pypdf import PdfMerger
merger = PdfMerger()
for pdf in ["file1.pdf", "file2.pdf", "file3.pdf"]:
merger.append(pdf)
merger.write("merged.pdf")
merger.close()Frequently Asked Questions
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