There was a time when you had to give your commercial printer a Quark document, along with the fonts and images required to produce your document. It was prone to error - they might not have supported your version of Quark and, when InDesign came along, wasn't supported at all; initially at least. Commercial printers soon realised that PDF documents enabled their customers to output their document - along with the fonts, layout and images required - and simply hand over for printing. They could then duplicate without error. Problem is for Windows users, there is no PDF support built in to the existing operating system and you'll need a PDF tool to convert your work to the format.31 Jan 2009