Privacy, without the fine print
There is no policy to read here, because there is no data to have a policy about. Nothing you open in fuckpdf is sent anywhere, and there is nowhere for it to be sent.
What happens to your file
You choose a file. Your browser reads it into memory and hands the bytes to an engine: PDFium, compiled to WebAssembly, in a Web Worker, or pdf-lib in the page itself. The engine produces a new file and the download comes from a blob in this tab. No step in that list touches a network.
How to check
Open your browser's developer tools, go to the Network tab, and run a tool. Nothing new appears. The site is served with a Content-Security-Policy of connect-src 'self', so the browser would refuse an outbound request even if the code tried to make one.
Offline
After the first load, tools you have already used keep working with no connection at all. Disconnect and try it. A site that processes files on a server cannot pass that test. That is what makes it worth running.
What is stored
One value in this browser's own localStorage: whether you picked the light or the dark theme. No cookies, no analytics, no telemetry, no error reporting, and no fonts or scripts loaded from anyone else.