Followed this tutorial to create PDF from scratch on iOS.
http://www.raywenderlich.com/6581/how-to-create-a-pdf-with-quartz-2d-in-ios-5-tutorial-part-1
Followed the post in StackOverflow to edit (overwrite onto) existing PDF on iOS as well.
http://stackoverflow.com/questions/1333710/how-to-edit-a-pdf-in-objective-c
If you tap "Create PDF", then it will generate PDF from scratch. If you tap "Edit PDF", then it will add contents on top of existing PDF (sample_0.pdf).