Now that I see what you are trying to accomplish, you could add a white rectangle and make it about 40-50% transparent.

I tried the page in Firefox, IE, Safari, Opera, and Chrome. Some links worked and some did not.

I wonder if the links that are not working are not actually linked?

Check each of the rectangles and make sure that each rectangle is linked, and make sure that the corresponding PDF file is in the index_html_files folder.