MSB normally exports as text for your use in other programs such as spreadsheets, but can export SMS conversations as HTML:
Versions before 3.0: Option– or ALT–click on the Export button.
Version 3.0 and greater: you are always prompted for your choice.
The conversation will then be displayed in a window in your default web browser; you can then Save As or print/save as a PDF. Your web browser may also allow selecting a different type size, if you wish.