uses ComObj;
      .....

   procedure TForm1.Button1Click(Sender: TObject);
   var
     NVmerge : Variant;
     retVal : Integer;
   begin
     NVmerge := CreateOleObject('Neevia.PDFmerge');
     retVal := NVmerge.MergePDF('c:\1.pdf+c:\2.pdf', 'c:\merged.pdf');
     if retVal <> 0 then
       Application.MessageBox(PChar('Error code=' + IntToStr(retVal)),'',0)
     else
       Application.MessageBox('Done','',0);
   end;