Generally, if you want to insert just a few lines of text in right-to-left mode, you should do the following:
DocumentBuilder builder = new DocumentBuilder(doc);
builder.ParagraphFormat.Bidi = true;
builder.Font.Bidi = true;
Run.Font.Bidi property sets the text direction in a run of text.
Paragraph.ParagraphFormat.Bidi sets the ordering of runs in paragraph and also inverts the meaning of ParagraphAlignment.Left and ParagraphAlignment.Right of ParagraphFormat.
To set text direction to right-to-left for all text in an already existing document use the following code:
NodeCollection paragraphs = doc.GetChildNodes(NodeType.Paragraph, true);
foreach(Paragraph paragraph in paragraphs)
paragraph.ParagraphFormat.Bidi = true;
NodeCollection runs = doc.GetChildNodes(NodeType.Run, true);
foreach(Run run in runs)
run.Font.Bidi = true;