To reproduce:

Create a word document with a watermark and export it to pdf as follows:

DocxFormatProvider dproivder = new DocxFormatProvider();
var doc = dproivder.Import(File.ReadAllBytes(@"C:\Users\gngeorgiev\Desktop\watermark.docx"));

PdfFormatProvider provider = new PdfFormatProvider();
var bytes = provider.Export(doc);
File.WriteAllBytes(@"C:\Users\gngeorgiev\Desktop\exp.pdf", bytes);

You will see that the watermark is missing.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.