A document feeder includes: a conveyance unit that separates one document from among documents placed on a document tray, and conveys the document; a first width-determining unit that determines a width of one of the documents; a second width-determining unit that determines a width of the conveyed document; a first length-determining unit that determines a length of one of the documents; a second length-determining unit that determines a length of the conveyed document; and a controller that if an inconsistency occurs between the width determined by the first width-determining unit and the width determined by the second width-determining unit, causes a display unit to display an operation guide including first content, and if an inconsistency occurs between the length determined by the first length-determining unit and the length determined by the second length-determining unit, causes the display unit to display an operation guide including second content.