Include \usepackage{color,hyperref} with Preamble for clickable link in blue. Compile with pdflatex. @unpublished{K:HSA, author = {Kettler, Paul Carlisle}, title = {Hilbert space analysis of integral equations}, note = {{Junior Paper, Department of Mathematics, Princeton University, William Feller, adviser}, \textcolor{blue}{\url{www.paulcarlislekettler.net/docs/HSA.pdf}}}, month = {Apr.}, year = {1962}}