For the time being we do not support HTML text in the TextBox item. The Web ReportViewer renders the text as expected since the browser actually do render the text (HTML). When printing or export a report in PDF/Image we use GDI and PDF which do not process the text as HTML but render it as a plain text.
This is a limitation that I believe we will soon overcome. As a workaround I can suggest you to strip the HTML tags thus leaving the plain text only. We have a function that you can use with any expression:=StripHtmlTags(Fields.HtmlTextField)
I hope this information helps.
the Telerik team