Yes when you have a WebForms control (with Runat="Server" attribute), you can't put <% ... %> code blocks inside that control tag. This is how ASP.NET WebForms markup (aspx) works. If you are trying to set GleamTech control's properties you can do it from code behind, e.g.
protected void Page_Load(object sender, EventArgs e)
{
documentViewer.Document = "...";
}If you are trying to insert something inside <head runat="server"> tag, then you can do this:
<head runat="server">
<asp:literal ID="someLiteral" run="server" />
</head>and then in code-behind you can set it like this:
protected void Page_Load(object sender, EventArgs e)
{
someLiteral.Text = "...";
}