RtfFormatProvider provider = new RtfFormatProvider();
RadFlowDocument document = provider.Import(File.ReadAllText(@"..\..\test.rtf"));
 
PdfFormatProvider pdfProvider = new PdfFormatProvider();
var tables = document.EnumerateChildrenOfType<Table>();

foreach (var table in tables)
{
    table.Borders = new TableBorders(new Border(BorderStyle.None));
}


using (FileStream stream = File.OpenWrite(@"..\..\result.pdf"))
{
    pdfProvider.Export(document, stream);
}