You should choose a suitable font like Nazanin. By this you get seprated farsi letters left to right.
Then you should change the writting direction from left to right to right to left.
use "PdfWriter.RUN_DIRECTION_RTL" in your settings.
PdfPCell and ColumnText have a property named runDirection that you should set it to PdfWriter.RUN_DIRECTION_RTL. In this way you can create a farsi pdf.