PDF or Portable Document Format is a popular file type that is often used for online documents. It's great for distributing downloadable written content, and is frequently used by governments and businesses alike. Because it's a format that's familiar to all, many applications allow the user to convert other document types to the PDF format. PHP is one programming language that has a built-in ability to convert to PDF. PHP scripts can be used to transform file types such as HTML into PDF files.