use a third-party library such as iTextSharp or PDFsharp. Here's an example using iTextSharp: 

Note that this code only adds the plain text contents of the RichTextBox control to the PDF file. If you want to include the formatted contents of the control (including font styles, colors, etc.), you'll need to use a different format, such as RTF or HTML. To convert the contents of the control in RTF format to PDF using iTextSharp, you can use the following code: 

we're using the RtfParser class of iTextSharp to parse the RTF code of the RichTextBox control and add the resulting contents to the PDF document. To convert the contents of the control in HTML format to PDF using iTextSharp, you can use the following code: