Why these tools run in your browser
Most online file tools upload your document to a server, process it there and ask you to download the result. That is slow for large files, it puts private documents on someone else's computer, and it is why so many sites limit file sizes or add queues. Every tool on this page works differently: it downloads the code to your browser and processes the file on your own device. Nothing is uploaded, there is no daily limit, and once a page has loaded it keeps working without an internet connection.
Each tool uses a well-known open-source engine. PDF tools use pdf-lib and qpdf, video and audio tools use ffmpeg compiled to WebAssembly, archive tools use libarchive and fflate, and the image resizer uses the high-quality scaling built into your browser. The tool pages explain exactly what each one keeps and what it changes, including the limits, so you know what to expect before you start.