If any security features are turned on by the creator of a PDF document, the PDF file will be encrypted. These security features let an author disallow printing, copying text/graphics, editing, and/or adding annotations.
As of version 0.91, xpdf includes decryption code. (The US government still requires open source software authors to report their URLs. This is silly, but not quite as bad as the previous situation.)
Thanks to Leo Smiers, Tony Nugent, and Felix von Leitner for providing (from outside the US) the source patch up through version 0.90.