Why are my pdf files printing with a black background and white letters like a photo negative?

In the Print dialog, disable "Print in grayscale" if selected, click on the "Advanced" button and make sure that the "Negative" option is not selected. If you can't deselect "Negative", change Color option from Composite to Composite Gray, for example, deselect Negative and change Color back to Composite. Click OK to confirm and you can select "Print in grayscale" again if necessary.