Most free "compress this image" or "convert this PDF" sites work the same way: upload your file to their server, process it, send it back. I built ToolHub to avoid that where possible — most of the 45+ tools on there run entirely in the browser, so the file never actually leaves your device. A ...
Source: [Dev.to](https://dev.to/trytoolhub/i-built-45-free-tools-that-process-files-entirely-in-the-browser-heres-the-architecture-58kh)