Oops. this is for a winforms app. Not for WPF. I will post this anyway.
try this
private AxAcroPDFLib.AxAcroPDF axAcroPDF1;
this.axAcroPDF1 = new AxAcroPDFLib.AxAcroPDF();
this.axAcroPDF1.Dock = System.Windows.Forms.DockStyle.Fill;
this.axAcroPDF1.Enabled = true;
this.axAcroPDF1.Name = "axAcroPDF1";
this.axAcroPDF1.OcxState = ((System.Windows.Forms.AxHost.State)(resources.GetObject("axAcroPDF1.OcxState")));
axAcroPDF1.LoadFile(DownloadedFullFileName);
axAcroPDF1.Visible = true;