You can handle the Pdf_Exporting
event and then check whether the generated output is XHTML
W3C Markup Validation Service
|protected void RadGrid1_PdfExporting(object source, GridPdfExportingArgs e)
| e.RawHTML; //copy the contents of this property
Alternatively you can send us a simplified working version of your project. In this way we will be able to debug it locally and provide more to-the-point answer for you.
the Telerik team