When document is Web layout mode (LayoutMode = Flow), table with LayoutMode = AutoFit is exported to RTF with wrong column widths. MS Word visualize such table splashed, and WordPad extremely large.

Workaround: Performing layout on the document in Paged layout mode seems to fix the issue:

            var doc = new XamlFormatProvider().Import(file);
            doc.LayoutMode = Telerik.Windows.Documents.Model.DocumentLayoutMode.Paged;
            doc.EnsureDocumentMeasuredAndArranged();
            doc.LayoutMode = Telerik.Windows.Documents.Model.DocumentLayoutMode.Flow;