protected void Page_Load(object sender, EventArgs e)
    {
        string userName = "user@Tenant.onmicrosoft.com";
        string password = "************";
        var securePassword = new SecureString();
        foreach (char c in password)
        {
            securePassword.AppendChar(c);
        }
        using (var ctx = new ClientContext("https://tenant.sharepoint.com/"))
        {
            ctx.Credentials = new Microsoft.SharePoint.Client.SharePointOnlineCredentials(userName, securePassword);
            Web web = ctx.Web;
            ctx.Load(web);
            ctx.ExecuteQuery();

            var file = ctx.Web.GetFileByServerRelativeUrl("/Shared Documents/test.pdf");
            ClientResult<Stream> data = file.OpenBinaryStream();
            ctx.Load(file);
            ctx.ExecuteQuery();

            HttpContext.Current.Response.ContentType = "application/pdf";
            HttpContext.Current.Response.AppendHeader("content-disposition", "inline;filename=" + file.Name);
            HttpContext.Current.Response.AppendHeader("content-length", data.Value.Length.ToString());
            HttpContext.Current.Response.BinaryWrite(StreamToByteArray(data.Value));
            HttpContext.Current.Response.Flush();
        }
    }

    private byte[] StreamToByteArray(Stream input)
    {
        byte[] buffer = new byte[16 * 1024];

        using (MemoryStream ms = new MemoryStream())
        {
            int read;
            while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
            {
                ms.Write(buffer, 0, read);
            }
            return ms.ToArray();
        }
    }