The file stays in the browser workflow
With a client-side tool, JavaScript reads the file selected on your device and performs the conversion in browser memory instead of first sending the file to the website server.
Libraries can still load from the web
A page may download JavaScript libraries from the site or a CDN. That is different from uploading the contents of the file you selected. Sensitive workflows should still be used on a trusted device and browser.
Device limits still apply
Large PDFs and high-resolution images can consume significant memory. If a browser tab closes or the device becomes slow, try a smaller batch or a desktop browser.