Error when deleting file. File is used by another process in ASP.Net

I tested this way and it works for me

 <a href = "Test.pdf" target = "_blank">PDF</a>
    <asp:Button ID="btnDelete" runat="server" Text="Delete" 
        onclick="btnDelete_Click" />

 protected void btnDelete_Click(object sender, EventArgs e)
    {
        File.Delete(Server.MapPath("~/Test.pdf"));
    }