The printing functionality comes from the browser. The RadEditor control provides only a suitable tool to invoke it. I would like to point out that this control is designed to be an XHTML editor and the functionality to print HTML document with certain paper size is not a matter that could be handled due to design matters. Note that the printing logic comes from the browser you can also investigate the native browser's print button and observe the same result.
HTML documents are intended to be rendered (drawn or printed) on browser, where the format of the area is calculated via the screen resolution, which is not related to the paper size standards.
For example MS Word is a desktop rich text editor, which can process text with formatting specifically to the set paper size and such feature is a required for such product. In the web environment there is no suitable approach to limit an HTML page to a certain paper size A4 format. Note that there is no suitable size option for an HTML document that could let the user to print it without breaking its layout. This is so because the printer client makes everything possible to fit the provided content.
Note also that the rich text editors are capable to control Page margins, footers, headers, page breaks etc., they are also vital for the set paper size and the text formatting, although such definitions are not suitable for the HTML documents and there is no technological reason for such to exist.
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