$ sudo apt-get install flpsed $ flpsed foo.pdf
Position the cursor by mouse and just start typing. If you don't like the position you can use the arrow keys for fine movements or grab it with the mouse. It doesn't have the most attractive user interface but who cares, it delivers.
You can also save the modified pdf.
Creative uses include printing something as a pdf like a "html print form" and leveraging the medium with tools like flpsed.