PDF offers a number of different security features:

  1. You can add a copy of your hand-written signature. This makes your PDF look nice but offers no protection.
    See Creating a signature
  2. You can limit what others people can do to your PDF. For example, you can stop editing or printing your PDF via a simple password. It may still be possible for someone to bypass this kind of protection if they know what they are doing. 
    See Security
  3. You can digitally sign your PDF. Anyone who receives a signed PDF can easily tell if it has been modified since it was originally signed. This form of guarantee is difficult to defeat and is commonly used for legal documents. 
    See Digital Signatures