You can insert a line break with the Alt+Enter keyboard shortcut. Programmatically this is achieved with the InsertLineBreak() method of RadRichTextBox or RadDocumentEditor.
Note that lines separated in this manner are still interpreted as part of the same Paragraph document element. What you should be aware of is that extremely large document elements can cause performance diminishment when scrolling and typing in the control, so I would not recommend replacing all paragraph ends with a line break. Instead, you can simply change the default spacing after of the Normal style:
.radRichTextBox.Document.StyleRepository.GetValueOrNull(RadDocumentDefaultStyles.NormalStyleName).ParagraphProperties.SpacingAfter = 0;
I hope this is useful.
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