What has broken when Preview no longer opens PDF documents? The answer is more complex than it might appear.
Desktop
Old archived files had lost association with the right app. What went wrong?
How can I sort out irritations in the appearance of my Mac’s Desktop?
Is it possible to prevent the individual Mac users on our network from changing the desktop background on their Macs?
With Mac OS 8 and 9 back in 2001, we had to performance routine maintenance to keep the Desktop working. This is how we did it.
The Eclectic Light Company 