I uploaded an HTML file that contains accentuated characters. They all come up with chinese symboles. Is there a way to customize this?

That is because OLW HTML Viewer always sets encoding of HTML file as UTF-8. One workaround is that you can save your HTML file as UTF-8. This can simply be done using Notepad. You can open the file and select "Save As" menu item. In the Encoding drop down list, select UTF-8.