The content area of RadEditor is an editable iframe/div element with applied contenteditable="true" and the copy and paste operation is handled by the browser.
Can you explain where do you paste the copied from RadEditor content: in RadEditor again, in a textarea, in MS Word, in another editable element or in other location?
What you can try is to not wrap the original content in <pre> tags, but to replace the line feeds with <br> symbols prior loading the content in RadEditor:
RadEditor1.Content = contentToBeLoaded.Replace(System.Environment.NewLine, "<br>")
You can find more information at https://stackoverflow.com/questions/4611591/code-vs-pre-vs-samp-for-inline-and-block-code-snippets
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