type
  TSearchThread = class(TThread)
  protected
    procedure Execute; override;
end;

var
  FormMain: TFormMain;
  SearchThread:TSearchThread;

[...]

procedure TSearchThread.Execute;
begin
  while not Terminated do
   begin
     SucheTextInDateien(FormMain.EditSuche.Text);
   end;
end;

procedure TFormMain.btnSucheClick(Sender: TObject);
begin
  if btnSuche.Caption='Abbrechen' then
   begin
     SearchThread.Terminate;
     btnSuche.Caption:='Suchen';
     FormMain.Status.Panels.Items[0].Text:='Suche abgebrochen !';
     exit;
   end;
  if trim(EditSuche.Text)='' then exit;
  btnSuche.Caption:='Abbrechen';

  SearchThread := TSearchThread.Create(false);
  SearchThread.FreeOnTerminate := true;
end;