Download PDF file from outside Root folder in ASP.Net

Refer the below sample code for your reference and implement it as per your code logic.

HTML

<div>
    <asp:LinkButton ID="lnkView" runat="server" Text="View PDF" OnClick="View"></asp:LinkButton>
</div>

C#

protected void View(object sender, EventArgs e)
{
    string path = Server.UrlDecode(@"C:\PDF\Test.pdf");
    if (!(new FileInfo(path).Exists))
        return;
    using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
    {
        SetStream(fs);
    }
}

private void SetStream(Stream stream)
{
    byte[] bytes = new byte[(int)stream.Length];
    string fileName = "fileName.pdf";
    stream.Read(bytes, 0, (int)stream.Length);
    Response.Clear();
    Response.Buffer = true;
    Response.ContentType = "application/pdf";
        
    Response.AddHeader("content-disposition", "attachment; filename=" + fileName);
    Response.BinaryWrite(bytes);
    Response.End();
}

VB.Net

Protected Sub View(ByVal sender As Object, ByVal e As EventArgs) Handles lnkView.Click
    Dim path As String = "C:\PDF\Test.pdf"
    If Not (New FileInfo(path).Exists) Then Return

    Using fs As FileStream = New FileStream(path, FileMode.Open, FileAccess.Read)
        SetStream(fs)
    End Using
End Sub

Private Sub SetStream(ByVal stream As Stream)
    Dim bytes As Byte() = New Byte(CInt(stream.Length) - 1) {}
    Dim fileName As String = "fileName.pdf"
    stream.Read(bytes, 0, CInt(stream.Length))
    Response.Clear()
    Response.Buffer = True

    Response.ContentType = "application/pdf"

    Response.AddHeader("content-disposition", "attachment; filename=" & fileName)
    Response.BinaryWrite(bytes)
    Response.End()
End Sub

Screenshot