An image forming system which is capable of enhancing security in an i-copy function. A server apparatus stores document data once having been printed in association with a sheet identifier recorded on a print sheet on which the document data has been printed. An image reader section reads the sheet identifier from the print sheet when a copy command is issued by the user authenticated based on an entered user identifier by a CPU of a MFP. A printer section reads out and prints the document data from the server apparatus. When the sheet identifier has not been read from the print sheet based on the copy command over a predetermined time period, the document data is inhibited from being printed by the printer section.