The ability to edit a PDF file is usually limited to paid programs. You can do so with services on the web that will allow you to do the same thing. The typical programs that you can download for free are just for reading the documents. If you actually want to edit documents outside of creating them on Word and saving them as a PDF, you are going to have to invest in a different type of software. There are some simple solutions if you want to edit PDF files on your computer today.
The Phat Photographer