How It Works
Why merging PDFs locally is safer and how the tool works.
How do I use it?
- Select multiple PDF files using the file picker.
- Reorder or remove files as needed.
- Click “Merge” to combine them into a single PDF.
- Download the merged file.
What happens when you merge PDFs?
When you select PDF files and click “Merge”, the app loads each file into your browser memory and combines the pages into a single PDF using WebAssembly (WASM).
WASM is a technology that allows high-performance code to run in the browser, enabling complex operations like PDF merging without relying on an external connection. Everything happens on your device so the files are never uploaded to a server.
Why merge PDFs offline?
- Privacy-first: Your documents never leave your computer.
- No accounts required: There’s nothing to sign up for.
- Fast and reliable: No network latency, and you can keep working even if you go offline.
How do the connection modes work?
Once the site is loaded, you can switch between online and offline modes at any time without affecting your files or progress, so long as you stay on the main page. Simply disconnect from the internet to enter offline mode, or reconnect to return online.
When you are online, the app may load advertising resources while blocking all other connections. These resources do not receive or process your PDF files. Offline mode keeps the core tool running without loading any external content.
You do not need to be offline to use the tool. The merging engine always runs locally and securely regardless of connection status.
What is WASM doing?
The PDF merge engine runs as a WebAssembly (WASM) module inside your browser. WebAssembly is a low-level binary format that lets high-performance code run safely inside web pages. In this app, the WASM engine reads your PDF files, stitches pages together, and generates the merged output without sending your data anywhere.
The WASM engine is what actually performs the heavy lifting of parsing and combining PDF pages. Your browser loads the WASM module once (from the site itself), and then the engine runs locally to process files.
How to monitor what’s happening
On the main page, there is an activity panel that you can open by clicking the magnifying-glass button in the bottom-right corner of the PDF merging dashboard. That panel shows what the WASM engine is doing and whether the site is making any network requests.
The panel also includes a small network indicator (online/offline) and a log of blocked/allowed connections. Use it to confirm that the merge is happening locally, and to see when the app blocks or allows network requests. This log is useful for verifying the engine is running properly and for validating that no unexpected traffic is being created.