Why can you create PDF files no matter the source you use? Because WinPDF acts like a printer. It installs itself on your computer and it will appear in your printer list when you'll try to print any file. Then if you want to create a PDF, you'll only have to choose WinPDF as printer and itwill give you your desired PDF file.
Thanks to that way of work, if the program you use allows you to print, it will allow you to convert the file into PDF. IT's extremely easy, if you can print, you can print PDF documents.