What is WebAssembly, and why does it let PDF tools skip the upload?

WebAssembly (usually shortened to WASM) is a format for running compiled code inside a web browser at close to the speed of a native app. It shipped in every major browser back in 2017, but it's only in the last few years that tools built on top of it have become common for everyday tasks.

Before WebAssembly, browsers could only really run JavaScript, which is fast enough for interactive pages but was never designed for heavy computation like decoding images, manipulating PDF structure, or running compression algorithms efficiently. Tools that needed that kind of processing had no local option — they had to ship the file to a server running C, Go, or Python, do the work there, and send it back.

WebAssembly changes the calculation. Libraries originally written in languages like C or Rust — including many of the same PDF and image-processing libraries that servers have used for years — can be compiled to WASM and run directly in your browser tab. The code is the same; only where it executes changes.

The practical result is what you're using right now: a merge, split, or compress operation that would previously have needed a server round-trip now happens on your own device, using your own CPU, in the time it takes to click a button. No file leaves your browser because there's no longer a technical reason for it to.

It's not magic, and it's not new — it's just now mature and fast enough that "runs in your browser" is a realistic default for tools like this one, instead of a compromise.