Acrobat files can be password protected so that, even with the file downloaded onto someone's computer, that person cannot extract, copy, or even print the contents of the Acrobat file unless they know the password.
You apply password protection to Acrobat files when you are saving them. There are two different types of password protection that you can apply. You can password protect the file so that people cannot even open the file unless they know the password. There is a second password that can be applied in addition or instead of the first that will allow people to read the file but which will no allow anyone who doesn't know this password from copying, updating, or printing the document.
To apply password protection to your file to protect the file contents you need to perform the following steps when saving your document.
Your file is now saved with the security options that you selected in effect. If you wish to do any of the functions that you have disabled, you will have to use the password that you entered to go back into the security options and uncheck the checked boxes.
With these security options in effect, you can safely distribute your document by whatever means knowing that the content of your document is safe from tampering. It is also almost impossible for anyone to steal your images - they would have to crack your password in order to do so.
To see this protection in action, take a look at this page which demonstrates the security measures described above.
If you don't have acrobat reader, it is available as a free download from the Adobe web site.
This article written by Stephen Chapman, Felgall Pty Ltd.