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