BaseFont tahoma = BaseFont.createFont("times.ttf",
BaseFont.IDENTITY_H, true);
 font  font = new Font(tahoma, 10);

Thanks for your attention, but

didn,t work

I think you get seperated letters  in words not sentences. Anyway, I guess you type your Persian text in a usual editor which gets your input all encoded between \u0600 and \u068f.

The characters mentioned above are uppercase characters which must be converted to apropriate lower case between the code \u0f70 and \ufeff to be shown correctly.

Editors like Microsoft Word save uppercases for farsi  but covert them using a complex script for showing them.

At last, take a look at the mentioned characters above in windows 'Character Map' then u can find out what happens.

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.