Image forming apparatus, method of controlling the same, and storage medium

A method of controlling an image forming apparatus having an image reading unit capable of conveying and reading a plurality of documents of different sizes includes determining whether an automatic magnification function is set in a case where an instruction to read a plurality of documents of different sizes is given, and performing control such that, in a case where the determining determines that the automatic magnification function is not set, a first document reading mode in which the plurality of documents is conveyed at a constant conveyance speed and is read is set to the image reading unit, and, in a case where the determining determines that the automatic magnification function is set, a second document reading mode in which the plurality of documents is conveyed at a different conveyance speed for each document and is read is set to the image reading unit.