PDF icons suddenly have a yellow triangle with an "!"...

suddenly my pdfs have a yellow triangle with an "!" in it as the default icon in the Finder (running 10.5.8). if I choose Get Info and change the default "Open with..." application from Preview to Adobe Acrobat, it's fine, the icon shows up as an Adobe Acrobat icon, but if I change the default program back to Preview the yellow triangle comes back. this was not the case a few days ago. not sure what I did to make this happen.