Eric wants to be able to lock a PDF file so that nobody can modify it. Leo says that every PDF creator, including Adobe Acrobat, can protect it so it can't be modified. He can also also set the PDF so that the document cannot be forwarded to anyone else. If he trusts the person that he's sending it to, he can encrypt the PDF so it requires a password to open it. But understand that if they can open it, they can make an image of it and share it outside of that. So he'll need to trust the person he sends it to.
Another PDF creator option is FoxIt.