Blogged from PC Pro Magazine
It’s worth noting that the PDF specification includes some security features that allow a document creator to forbid others from doing certain things, such as saving, printing or editing a PDF. Should you need to make an edit to a locked file, a quick web search should turn up plenty of free tools and websites that will unlock PDF files for free.
Alternatively, as long as the document isn’t locked for printing, there’s another trick that can be used – so long as you have both a PDF creation tool and the Microsoft Microsoft XPS Document Writer installed on your system as virtual printer drivers. All you need to do is print the document using this latter driver, to create a copy of it in XPS format – then open this document in Reader or another application and reprint it as an unprotected PDF. You should then be able to tamper with it as you choose.