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; storing at least a first portion of the PDF document in the memory if a determination is made that the size of the PDF document is larger than the available capacity of the memory; transmitting a remaining portion of the PDF document to at least one external device if the determination is made that the size of the PDF document is larger than the available capacity of the memory; receiving the remaining portion of the PDF document from the at least one external device; and forming an image corresponding to the stored at least first portion of the PDF document and the received remaining portion of the PDF document.