Use PDF security features to lock access
It is sometimes useful to save documents not in their native form, but in PDF format instead. This is Adobe's Acrobat file format, but it is so widely supported by all operating systems that it is a good way to distribute documents.
It ensures that people can view your document correctly no matter what computer, operating system, software or fonts they have.
In some cases you might want to limit the people that can access a document and it is possible to password protect it so it can't be opened by unauthorised people.
In other situations you may want to let people open it and read the contents, but not to be able to copy the text and images out of it. You might even want to prevent people from printing it too.
All these things are possible with the security features in PDF files, but accessing them isn't quite as straighforward as you might think. It depends on how you go about it.
To see an example of this, start TextEdit, enter some text to create a document and then select Save As PDF (now Leopard) or Export as PDF (Lion). You'll get a standard Save As dialog in which you can enter a filename and select the folder to save it. Where are the security options? There aren't any!
Now try this: Select File, Print. There's no need for a real printer and you can click the PDF button in the bottom left corner of the dialog and select Save As PDF. You'll get a much better Save dialog and it has lots of useful features. Now you can enter the title, author, subject and keywords.
Click the Security Options button and you can require a password to open the document, require a password to copy text, images or other content, and require a password to print a document.
That's much more useful and it's a better way to create PDFs than the Save As PDF menu.
What if you already have unprotected PDFs that you would like to protect? If you double click a PDF file, it will open in Preview. You can then choose File, Save As and you'll see an Encrypt option in the Save dialog.
It's sort of half way between the no-option Save As PDF in TextEdit and the full security features you get when you print to a PDF. If you want the full features, then choose File, Print in Preview as in TextEdit.