The ParagraphDefaultSpacingBefore property is 0 by default and LineSpacing is responsible for the space between lines in the same paragraph, so you do not need to set them in order to achieve the wanted behavior. However, I am not sure why you are unable to change the spacing after, as putting this code:
.editor.Document.ParagraphDefaultSpacingAfter = 0;
in the constructor of the page and in the DocumentChanged event handler results in the needed behavior on our end as you can verify from the attached picture.
On the other hand, this does not change the structure of the document and each time you press Enter, that will result in a new paragraph and not a line break (or return). What you can do in this regard is subscribe to the PreviewEditorKeyDown
event of RadRichTextBox and suppress the default action like this:
sender, Telerik.Windows.Documents.PreviewEditorKeyEventArgs e)
(e.Key == Key.Enter)
When pressing Enter, the space will be equal to the one between lines in the same paragraph and the document will not break into separate paragraphs. You can refer to the attached image for a reference.
Please not that it is not recomended to create large paragraphs in RadDocument for performance purposes
I hope the provided information is helpful! Let us know if you have other questions.