Drop in a password-protected file — a Form 16, say — enter its password, and get back the same document with no password required. Nothing is uploaded anywhere; the file never leaves this browser tab.
How it works. This page loads MuPDF compiled to WebAssembly and runs the decryption 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.