Every other free compressor uploads your document to a server and asks you to trust a retention policy. This one has nothing to promise : the work happens on your machine, so there is no copy to keep.
The file is read straight from your disk into the page. Open your browser network tab and watch : no request is made.
We never receive the document, so there is no retention window, no deletion timer, and nothing of yours in a breach.
Turn off your wifi and compress anyway. The whole tool is a few hundred kilobytes of code running in your tab.
It depends entirely on what is inside the file, and any tool that promises a fixed percentage is guessing. Measured on real documents :
If a level would make your file bigger, you get the original back instead. This tool never hands you a worse PDF than the one you gave it.