I don't see any argument on the xpdf site that justifies the restrictions on what may be done with a PDF. There is this:
"...it would be very hypocritical of me to, on one hand, ask people to honor my licensing restrictions, and, on the other hand, bypass (or assist others in bypassing) another author's requested restrictions."
But this is a flawed argument, because the xpdf author's licensing restrictions (GPL) are not the same as the document author's requested restrictions (no copying, printing. etc.). It's not hypocrisy if I ask you not to smoke in my house, but then eat doughnuts in your house against your express wishes. You could argue that it's inconsiderate, but how much that matters has to be weighed against the reasonableness of the requested restriction. I imagine most people here would agree that the GPL's restrictions are reasonable, but that trying to prevent someone from copying text from or printing a PDF is just dumb.
The xpdf author also says:
"In addition to all of this, Adobe requires that implementors of the PDF spec adhere to the document permissions."
But as has already been pointed out, blindly following a spec just because it's a spec is also dumb.