how to reduce pdf file size using c# windows form

Question

i am using this code but 


http://www.dotnetspark.com/kb/1807-pdf-compression-by-itextsharp.aspx


but

Facing this Error 

An unhandled exception of type 'System.ArgumentException' occurred in itextsharp.dll

Additional information: PdfReader not opened with owner password


Pelase help me