If I send an HTML file to someone by email, they can always be a little prudent and hover over a link before clicking. A tooltip shows them the destination URL, and this gives them a chance to know where the link will take them before actually clicking.
When I send a PDF file, the link remains burried. A recipient has no way of evaluating whether they are already familiar with the destination before going there, whether they are interested, etc.
\documentclass{article}
\usepackage[colorlinks=true]{hyperref}
\begin{document}
\huge\href{http://google.com}{Google} it or
\href{http://bing.com}{Bing} it.
\end{document}
Is it possible to add a tooltip to a PDF link? A solution that works on just Acrobat Reader is a good start. One that also works on OS X Preview is better.