1. A printing apparatus comprising: a reading unit configured to perform reading at a reading position and then to generate image data; a printing unit configured to print an image on a sheet based on the image data generated by the reading unit; a determining unit configured to determine whether image data includes an abnormal pixel, the image data being generated by the reading unit with no document placed at the reading position; and a control unit configured to control the determining unit to perform the determination in a case where a type of the sheet on which the image is to be printed by the printing unit is a first type of sheet, and to control the determining unit not to perform the determination in a case where the type of the sheet on which the image is to be printed by the printing unit is a second type of sheet which is different from the first type of sheet.