Why I am unable to download file with xls,pdf etc from webserver showing css issues?

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VS2005

C#2.0

C#

IIS

Visual-Studio

Why I am unable to download the files(xls,pdf etc). On downloading I am getting issue as Missing file (C:\Documents and Settings/Gridstyle.css,Stylesheet.css etc issues on downloading a word file,unsupported format even with xls extension is opened and adobe reader not in supported type etc.

String ext = String.Empty;
           if (strFile.IndexOf(".") > 0)
               ext = (strFile.Split('.'))[1];

           if (ext.ToLower() == "txt")
               Response.ContentType = "text/html";
           if (ext.ToLower() == "xls")
               Response.ContentType = "xls/html";
           if (ext.ToLower() == "pdf")
               Response.ContentType = "application/pdf";

           ////combine the path and file name
           if (File.Exists(strFile))//open the file and process it
           {
               FileStream sourceFile = new FileStream(strFile, FileMode.Open, FileAccess.Read);
               long FileSize;
               FileSize = sourceFile.Length;
               Response.AddHeader("Content-Length", FileSize.ToString()); //*
               byte[] getContent = new byte[(int)FileSize];
               sourceFile.Read(getContent, 0, (int)FileSize);

               sourceFile.Close();
               Response.BinaryWrite(getContent);
           }