Hi Vaks,
Thank you for your interest in syncfusion products.
The background to the Pdf document can be applied using DrawWatermark method.
The sample code snippet
String image = Server.MapPath (@"Data\Sunset.jpg");
pdfDoc.LastPage.Graphics.DrawWatermark (image, WatermarkImageType.Stretched);
//Draw some content on the page.
System.IO.StreamReader myFile = new System.IO.StreamReader(Server.MapPath(@"Data\SampleText.txt"));
String read = myFile.ReadToEnd ();
Please refer the sample in the below link
http://websamples.syncfusion.com/samples/PDF.Web/5.2.0.25_Pdf.Legacy/39017/main.htmPlease let me know if you have any questions.
Thanks,
Prakash