I do not want to create new PDF file,that I had already done but want to show and edit existing pdf file in iOS through code..
Is this possible or not and if possibl
You can use Quartz2D to manipulate your PDF, open them, make transformation and so on.
Check out the official documentation
Update :
Create your pdf context as explained in the documentation, then you just have to write text :
http://developer.apple.com/library/ios/#documentation/graphicsimaging/conceptual/drawingwithquartz2d/dq_text/dq_text.html#//apple_ref/doc/uid/TP30001066-CH213-TPXREF101