public ExportOptionsBase ExportOptions { get; set; }

Public Property ExportOptions As ExportOptionsBase

<dx:ASPxGridView ID="ASPxGridView1" runat="server" OnBeforeExport="ASPxGridView1_BeforeExport">
</dx:ASPxGridView>

protected void ASPxGridView1_BeforeExport(object sender, DevExpress.Web.ASPxGridBeforeExportEventArgs e) {
    switch(e.ExportTarget) {
        case ExportTarget.Pdf:
            var opts = e.ExportOptions as PdfExportOptions;
            opts.PasswordSecurityOptions.PermissionsPassword = "MyCustomPassword";
            break;
        ...
    }
}