PdfFormatProvider creates an invalid PDF document. 

Workaround:

RadDocument radDocument = null;
XamlFormatProvider xamlformatProvider = new XamlFormatProvider();
using (FileStream inputStream = new FileStream("input.xaml", FileMode.Open))
{
    Console.WriteLine("reading input file");
    radDocument = xamlformatProvider.Import(inputStream);
}

var provider = new Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
var bytes = provider.Export(radDocument);

var provider2 = new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
var flowDoc = provider2.Import(bytes);

var provider3 = new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();

using (Stream output = File.OpenWrite(@"output.pdf"))
{
    Console.WriteLine("writing output file");
    provider3.Export(flowDoc, output);
}

Console.WriteLine("done...");
Console.ReadKey();

The paste options popup stays visible when the window is deactivated

Workaround: 

Please see the attached video.

I've managed to fix this by altering the source code of  method ParagraphLayoutBox.MeasureOverride and hope it doesn't cause other problems.

The non-breaking space breaks the word if a comma character is presented in the word. For example, the following text should be treated as one word - 10,0 % but it is separated into to words - 10,0 and %.

The issue can be observed when the word should break on the next line. 

Text containing mixed Right-to-Left and Left-to-Right parts are visualized incorrectly - some of the punctuation marks swap their places.

Partial workaround: Use RadRichTextBox's TextRenderingMode = TextBlockWithPropertyCaching. This mode also has problems, but it works in some cases.