Use ctrl-D to get to the document properties and then select the security tab. You have two types of security options. One requires a password to even open the file. The second uses a password to set restrictions on copying, printing, etc. Be aware that this security works reasonably for most users, but there are work arounds that defeat your desired result.
Your best protection may be a legal one in which you specifically copyright the information in the file, possibly on top of the security settings. People may copy the information, but then you have a legal alternative to use to get compensation -- though not a simple process and may not be worth the trouble. The most secure step is to use digital rights management. However, even DRM is not perfect and often annoys folks because they can't even transfer the file to another computer in some cases (at least this is the case for music files).
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