Strip the password from an encrypted PDF and get back an unrestricted copy. You must supply the current password - this decrypts a document you already have access to, so that you stop having to type the password every time you open it, or so it can be passed to a tool that cannot read encrypted files. It is not a password recovery tool and cannot open a file whose password you do not know.
No. The correct password is required to decrypt the file, and nothing here attempts to guess or break it.
Most operations cannot read an encrypted document at all, which is why merging or converting one fails. Remove the password first, then run the tool you wanted.
Decrypting produces a document without the restrictions attached. To change permissions while keeping the file encrypted, use Change PDF Permissions instead.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.