Include \usepackage{color,hyperref} with Preamble for clickable link in blue. Compile with pdflatex.
@unpublished{K:TSIQE,
author = {Kettler, Paul Carlisle},
title = {A transformation to solve indefinite quadratic equations in integers},
note = {Preprint \#9, Pure Mathematics, Department of Mathematics, University of Oslo, \textcolor{blue}{\url{www.paulcarlislekettler.net/docs/FQT.pdf}}},
month = {Jun.},
year = {2007}}