Thanks for the support. I am able to achieve what I want. i.e high quality charts in the PDF and Excel documents. I have noticed that the following procedure helps in achieving the requirement. Save the chart in .emf form using SaveImage method and then draw the image on the document using the following code snippet . This procedure is producing high quality charts only when saved in .emf format, there is no difference in other formats.
PdfDocument pdfDoc = new PdfDocument();
PdfPage page = pdfDoc.Pages.Add();
PdfGraphics g = page.Graphics;
PdfImage image;
Stream imgStream;
imgStream = new FileStream("D:\\Syncfusion\\Chart.emf", FileMode.Open, FileAccess.Read);
image = PdfImage.FromStream(imgStream);
g.DrawImage(image, leftX, heightY);