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 and/or copying text.
Unfortunately, the US government has (rather silly) export controls on cryptography software. This means I can't legally provide a version of xpdf that can decrypt encrypted PDF files.
Luckily, the Internet is not limited to the US (despite what certain of our government bureaucrats may believe). Thanks to Leo Smiers, Tony Nugent, and Felix von Leitner, a source patch for xpdf is available.