I'm currently working for PDF Generator application in C# using iTextPDF library for my organization. How can I make a forty degree rotation to a text inside PDFPCell object. Can I do some tricks for making it work instead of manually using PDFContentByte method, such as ShowTextAligned.
I also provide the sample image about my letter criteria. Thanks.

This depends on the iText(Sharp) version you use.
In version 5.x and earlier you either have to use
ShowTextAlignedor you have to manipulate the content stream before and after writing the text to rotate the coordinate system. For drawing the text in a very high level API, therefore, you would pay by very low level content stream manipulation.In version 7, on the other hand, arbitrary rotation of cell contents is possible in the high level API. For example you can do it like this (for iText/Java; porting to iTextSharp/.Net should be trivial):
(RotateCellContent.java)
This results in
This been said, though, your sample image seems to indicate that you simply want to repeat that rotated text again and again as a background. In this case a solution as suggested by @Bruno in his comment might be more apropos.
This would have the additional advantage that the text would not get in the way of copy&paste / text extraction.