Are my files uploaded anywhere?

When you open a tool, your browser downloads the processing code, the same way it downloads the page. Your file is then read and processed locally, inside your browser. The result is created on your device and saved straight to it.

Check it yourself

Open your browser's developer tools, go to the Network tab and process a file. You'll see the page and its code load, but no request carrying your file. The How it works page explains this step by step.