Error trying to use a pdf file located in my project folder

Question

I am making an application which inserts text into a pdf template, but I am encountering a problem and it is that when I want to use the template it is as if I could not find it, so I did a validation with File.Exists, and it effectively returns me false, but when I look for the same path in the file explorer it finds me the pdf without problems. This is my code:

protected void btn_Guardar_Click(object sender, EventArgs e)
{
    try
    {
        string newName = $"numdoc_{ddl_TipoCapacitacion.SelectedValue}-{lastId.Rows[0]["ID"]}-{DateTime.Now.ToString("dd/MM/yyyy").Replace("/", String.Empty)}.pdf";
        file = Path.Combine(location, newName);
        string oldFile = string.Concat(Server.MapPath($@"~/plantillasPdf/capacitaciones/plantillaDiploma.pdf"));
        string newFile = string.Concat(Server.MapPath($@"~/plantillasPdf/capacitaciones/{newName}"));
        if (!File.Exists(oldFile))
        {
        }
        PdfReader reader = new PdfReader(oldFile);
        ...
    catch (Exception ex)
    {
        ControlarExcepcion(ex);
    }
}