What Harsh says is the best way to do that. However, be aware that there are various ways that editing can still be done. For example, not all PDF applications will respect a password. So, for example, if a person wants to, they can print the document out and then scan and OCR the document.
Simply securing that a document cannot be edited is no more "secure" than any lock — with motivation and time, nothing is ABSOLUTELY secure. I just want to make sure you know this going into your project.