[darkamerico]

Este otra versión de la función, interesante pero también presenta error:

Código Delphi [-]
procedure TfrmPrincipal.ViewPDFInWebBrowser(WebBrowser: TWebBrowser; const Base64Str: string);
var
  PDFBase64: TBase64Encoding;
  PDFDoc: Variant;
  HTMLString: WideString;
  Stream: TStringStream;
begin
  PDFBase64 := TBase64Encoding.Create;
  try
    PDFBase64.Decode(Base64Str);
    PDFDoc := '' +
              '<_embed width="100%" height="100%" type="application/pdf" ' +
              'src="data:application/pdf;base64,' + Base64Str + '">' +
              '';
    HTMLString := PDFDoc;
    Stream := TStringStream.Create(HTMLString, TEncoding.UTF8);
    try
      WebBrowser.Navigate('about:blank');
      while WebBrowser.ReadyState < READYSTATE_INTERACTIVE do
        Application.ProcessMessages;
      (WebBrowser.Document as IPersistStreamInit).Load(TStreamAdapter.Create(Stream));
    finally
      Stream.Free;
    end;
  finally
    PDFBase64.Free;
  end;
end;