Drop in a PDF, choose a password, and get back the same document protected with real AES-256 encryption. Nothing is uploaded anywhere; the file never leaves this browser tab.
How it works. This page loads MuPDF compiled to WebAssembly and encrypts the PDF (AES-256) in your browser's memory — the same engine desktop PDF tools use, just running locally instead of on a server. First load pulls down about 10 MB of engine files; after that it works from cache.