Maybe you saw the news item – classified information about the shooting of an Italian intelligence agent in Iraq was found by journalists in a PDF document given to the media by the US military.
Here’s an article with more information about why the text was not secured in the PDF file. The military had blacked out parts of the document, but had not removed the text underneath or properly secured the file.
The security in Adobe Acrobat is confusing. Although the perception is that PDF files cannot be changed, the reality is that Acrobat files can be edited in ways that are difficult to track. If you are dealing with sensitive material, you need to spend some time learning Acrobat’s security system!