Graebert of Germany has developed a secure version of DWF. The Quick seCure Format (QCF) format uses strong encryption (RSA and AES) to encrypt and password-protect DWF files.
To create QCF files -- use one of Graeberts (GiveMePower) software products: FelixCAD 5, FelixCAD LT, PowerCAD Pro 6, PowerCAD CE Classic, or PowerCAD CE Pro. It costs an extra e995 (US$1,200), however, to active the feature in those desktop and handheld CAD packages.
To view QCF files -- use the free DwfView software (available also for handheld computers), and provide the password.