[Rat]

Всё. Понял. Сделал так:

Код:

procedure TMainForm.ToolButton3Click(Sender: TObject);
var
  i : Integer;
begin
  SaveDialog1.FileName := PageControl1.Pages[PageControl1.ActivePageIndex].Caption;

  if(SaveDialog1.Execute) then
  begin
    for i:=0 to PageControl1.ActivePage.ComponentCount-1 do
      if  PageControl1.ActivePage.Components[i] is TRichEdit then
      begin
        (PageControl1.ActivePage.Components[i] as TRichEdit).Lines.SaveToFile(SaveDialog1.FileName);
      end;
    end;
end;