On most Macintosh computers when a link to a pdf file on a web site is clicked, the pdf file is saved to the user's hard drive and then opens. This causes any relative links on the pdf file to not work. Since the base maps for the aerial photos use relative links to enable a user to open a specific photo this poses a problem.
To overcome this problem, Macintosh users can use a plug-in that loads pdf files within a web browser. The plug-in is free download (other than for commercial enterprises) and is available at http://www.schubert-it.com/pluginpdf/ - you must be running Mac OS 10.3 or higher and should logout or re-start after loading the plug-in.