The problem occurs because of the <table> element that is added to the content. RadEditor is using third party tool to provide the HTML to PDF conversion and this tool requires table elements to have set width and to have <colgroup>. This is handled by the ExportToPdf filter along with other requirements of that tool, however, since you are dynamically setting the content server-side, content filters are not executed.
In order to dynamically wrap the content in table when exporting you need to modify the table declaration to match the following:
sender, ImageClickEventArgs e)
"<table style=\"width:0px\"><colgroup><col /></colgroup><tr><td>"
+ aa +
Please note that you need <col /> element for each column of the table.
the Telerik team