The easiest way to check if a "private" web tool is actually private: turn off your wifi
There's a whole category of browser tools (PDF converters, image editors, redaction tools) that advertise "files never leave your device" or "100% local, in-browser." Some genuinely do the work in your browser with WebAssembly. Plenty just say they do.
You don't need DevTools or any technical skill to check. Load the page, disconnect your internet, and try to actually use it.
- Still works fully offline: the processing is happening on your machine. Your file physically cannot have left; there's no connection for it to leave through.
- Breaks the moment you go offline: it needs a server to do the job. That doesn't automatically mean it's uploading your file (could just be loading code); but you can't rule it out anymore, and you're back to trusting their word.
I ran this on a handful of popular "local" PDF tools recently. Most stopped working the second I disconnected. A couple passed. Not naming names (I don't want to start a brigade or send anyone traffic): the point is the test, not a list. Run it yourself on whatever you use.
This is the one privacy claim you can verify with zero tools and zero trust. Pull the plug. Either it works or it doesn't.