Add PDF mention

free-programming-books.md

@@ -1351,7 +1351,7 @@ See also [TeX](#tex)
 * [Invent Your Own Computer Games With Python](http://inventwithpython.com/chapters/) - Al Sweigart
 * [Learn Python The Hard Way](http://learnpythonthehardway.org/book/)
 * [Learning to Program](http://www.freenetpages.co.uk/hp/alan.gauld/)
-* [Learning to Program with Python](http://python.cs.southern.edu/pythonbook/pythonbook.pdf) - Richard L. Halterman
+* [Learning to Program with Python](http://python.cs.southern.edu/pythonbook/pythonbook.pdf) - Richard L. Halterman (PDF)
 * [Lectures on scientific computing with python](https://github.com/jrjohansson/scientific-python-lectures) - J.R. Johansson
 * [Making Games with Python & Pygame](http://inventwithpython.com/pygame/chapters/) - Al Sweigart
 * [Modeling Creativity: Case Studies in Python](http://www.clips.ua.ac.be/sites/default/files/modeling-creativity.pdf) - Tom D. De Smedt (PDF)