I've done a lot more digging... I may have mis-lead a little bit since we have to go through so much trouble to convert from RadDocument to RadFixedDocument...

I've deduced it down to being the Fixed.Model.Editing.Block measures to a much greater width than it should be when text is inserted. So the "offset" bug is in fact on our side, BUT it's an offset based off the increased difference in Block size, which seems to be on Telerik's end.

Here's some sample code to distinguish the problem:

      internal static void Export(RadRichTextBox textBox)
      {
         BuildBlock(textBox.Document.EnumerateChildrenOfType<Span>().First());
      }

      private static void BuildBlock(Span span)
      {
         var block = new Telerik.Windows.Documents.Fixed.Model.Editing.Block();
         block.TextProperties.FontSize = span.FontSize;
         block.InsertText(span.FontFamily, span.FontStyle, span.FontWeight, span.Text);
         var textWidth = span.GetAssociatedLayoutBoxes().Aggregate(0.0f, (sum, box) => box.BoundingRectangle.Width + sum);
         var blockWidth = block.Measure().Width;
         // Assert.That(blockWidth, Is.InRange(textWidth - 0.001, textWidth + 0.001));
      }

You can set a button control to call this with a RadRichTextBox width 330, height 100, Font Segoe UI, Font Size 24, Text 私は測定を散会に分けて

        <telerik:RadRichTextBox
            x:Name="radRichTextBox"
            Width="330"
            Height="100"
            VerticalScrollBarVisibility="Disabled">
        </telerik:RadRichTextBox>
        <Button Grid.Row="1" Content="Export to PDF" Click="Button_Click"/>

The Assert should be around 319 for both... for a font family like Meiryo it's the same, but not for Segoe UI.

Hope this helps!

Yep! Not sure why the image got broken. Trying to add it again and also attaching it as a file.

You can just type this text into a plain document with just RichTextBox with a width of 250 height 100 (fontsize 20 font Segoe UI)

私は測定を散会に分けてやるつもりです