I repair printers for a living. The lines on the copied pages that only appear when using the automatic document feeder (ADF) is because the scan glass has crud on it. Lift the lid and you will see a narrow, 1/2 wide inch or so, glass to the left of the main glass. This is the area where the ADF scan takes place. A rag or paper towel, with rubbing alcohol, will remove the mark causing the line. Look at the position of the line, and it will correspond to spot where the crud or ink spot is.
I break printers for a living.
Let's start a company!