What does PDF stand for in file extensions?

PDF stands for Portable Document Format. The PDF was an attempt by Adobe to make a "standard" document format that could be read by all computers. The idea was sound but caused confusion, as you needed Adobe's software and plugins to even just look at the file.

The PDF is still widely used today, especially on the internet and since Word 2007 went on sale, as it now includes an "Export as PDF" option.