I am using:
Response.Clear();
Response.ContentType = "pdf";
Response.AppendHeader("Content-Disposition", "attachment; filename=test.pdf");
Response.WriteFile(this.txtFileName, true);
Response.Flush();
Response.End();
which shows a prompt to download the file and the prompt by default has “Open”, “Save” and “Cancel” buttons with “Cancel” selected by default.
Is it possible for me to hide the “Open” button and force users to eitehr save or cancel?
This is a standard dialog provided by the browser. As far as I know there is no way to mess with it and there is also no reason to do that.
You should probably try to educate your users on how to use the browser (the problem with opening a large file is quite common, usually this ends up in some temp folder anyway), instead of trying to change its behavior.