or download this
\documentclass[pdftex, a4paper, 11pt]{book}
\usepackage[left = 3cm, right = 3cm, top = 2.5cm, bottom = 2.5cm]{geom
+etry}
\usepackage[bookmarks, colorlinks, plainpages=false, pdfpagelabels]{hy
+perref}
...
\bibitem{example} This is an example
\end{thebibliography}
\end{document}