A document feeder includes a document placing tray on which a document is placed; a document discharging tray on which a document discharged from a document discharging port in a discharging direction is stacked; and a light source that radiates light to the document discharging tray. The document discharging tray is disposed below the document placing tray. The document discharging tray has a protruding portion protruding upward from the surrounding area. The light source is disposed within a region having a width that is the same as a document having a maximum size in the width direction.

There are provided a sheet feeding apparatus and an image forming system which can horizontally maintain a posture of a topmost envelope in stored envelopes, by a simple configuration. The sheet feeding apparatus includes: a stacking surface portion 321; a sheet feeding roller 313; a pressing surface portion 362; and a curve forming portion 363. A length of the stacking surface portion 321 in a width direction is shorter than a length of each of envelopes P in a width direction. The pressing surface portion 362 is pressed by the sheet feeding roller 313 via the envelopes P. The curve forming portion 363 is arranged outside the sheet feeding roller 313 in a width direction, projects more upward in a vertical direction than the pressing surface portion 362, and supports the plurality of stacked envelopes P with a predetermined size.

A sheet stacking tray includes a first stacking portion, a second stacking portion, and a base portion. In a sheet widthwise direction perpendicular to the sheet discharge direction, the first stacking portion includes a first sheet stacking surface having a first stacking width within a range from one half of a sheet width of a maximum-size sheet to be discharged up to the sheet width of the maximum-size sheet. The second stacking portion includes a second sheet stacking surface having a second stacking width smaller than the first stacking width. A border between the first stacking portion and the second stacking portion lies on a discharge-direction upstream side of a contact point where a leading end portion of the maximum-size sheet comes into contact with the sheet stacking part while the sheet is discharged.