1. A PDF (portable document format) document direct printing method of an image forming apparatus having a memory, the method comprising: comparing a size of the PDF document with an available capacity of the memory; transmitting the PDF document that is larger than the available capacity of the memory to at least one external device if a determination is made that the size of the PDF document is larger than the available capacity of the memory; receiving the PDF document from the external device; and forming an image corresponding to the received PDF document.