Как сделать поиск/замену в документе?

You should use a variant because the Find.Execute method is a bit buggy. Something like this, for example:

{ ... }
var
  Rnge: OleVariant;
{ ... }
 
Rnge := Doc.Content;
Rnge.Find.Execute('old', Wrap := wdFindContinue, ReplaceWith := 'new', Replace :=
  wdReplaceAll);
{ ... }

{ ... }
  { Create the OLE Object }
WordApp := CreateOLEObject('Word.Application');
WordApp.Documents.Open(yourDocFile);
WordApp.Selection.Find.ClearFormatting;
WordApp.Selection.Find.Text := yourOldStr;
WordApp.Selection.Find.Replacement.Text := yourNewStr;
WordApp.Selection.Find.Forward := True;
WordApp.Selection.Find.Wrap := 1; {wdFindContinue}
WordApp.Selection.Find.Format := False;
WordApp.Selection.Find.MatchCase := False;
WordApp.Selection.Find.MatchWholeWord := False;
WordApp.Selection.Find.MatchWildcards := True;
WordApp.Selection.Find.MatchSoundsLike := False;
WordApp.Selection.Find.MatchAllWordForms := False;
WordApp.Selection.Find.Execute(Replace := 2); {wdReplaceAll}
{Or as alternative:  WordApp.Selection.Find.Execute(Replace := 1); for one replace}
WordApp.ActiveDocument.SaveAs(yourNewDocFile);
WordApp.Quit;
WordApp := Unassigned;
{ ... }

Взято с Delphi Knowledge Base: https://www.baltsoft.com/

{ **** UBPFD *********** by kladovka.net.ru ****
>> Заменить строки в файле Word
 
Функция заменяет файлы в документе word с опциями поиска и замены
 
Зависимости: ComObj
Автор:       [NIKEL], nikel@pisem.net, Norilsk
Copyright:   Some help
Дата:        15 сентября 2002 г.
********************************************** }
 
uses ComObj; 
 
// Флаги замены 
type 
  TWordReplaceFlags = set of (wrfReplaceAll, wrfMatchCase, wrfMatchWildcards); 
 
function WordStringReplace(ADocument: TFileName; SearchString, 
         ReplaceString: string; Flags: TWordReplaceFlags): Boolean; 
const 
  wdFindContinue = 1; 
  wdReplaceOne = 1; 
  wdReplaceAll = 2; 
  wdDoNotSaveChanges = 0; 
var 
  WordApp: OLEVariant; 
begin 
  Result := False; 
 
  { Существует ли файл } 
  if not FileExists(ADocument) then 
  begin 
    ShowMessage('Файл не найден!'); 
    Exit; 
  end; 
 
  { Создаем OLE объект } 
  try 
    WordApp := CreateOLEObject('Word.Application'); 
  except 
    on E: Exception do 
    begin 
      E.Message := 'Word недоступен'; 
      raise; 
    end; 
  end; 
 
  try 
    { Прячем Word } 
    WordApp.Visible := False; 
    { Открываем документ } 
    WordApp.Documents.Open(ADocument); 
    { Инициализируем параметры} 
    WordApp.Selection.Find.ClearFormatting; 
    WordApp.Selection.Find.Text := SearchString; 
    WordApp.Selection.Find.Replacement.Text := ReplaceString; 
    WordApp.Selection.Find.Forward := True; 
    WordApp.Selection.Find.Wrap := wdFindContinue; 
    WordApp.Selection.Find.Format := False; 
    WordApp.Selection.Find.MatchCase := wrfMatchCase in Flags; 
    WordApp.Selection.Find.MatchWholeWord := False; 
    WordApp.Selection.Find.MatchWildcards := wrfMatchWildcards in Flags; 
    WordApp.Selection.Find.MatchSoundsLike := False; 
    WordApp.Selection.Find.MatchAllWordForms := False; 
    { Ищем} 
    if wrfReplaceAll in Flags then 
      WordApp.Selection.Find.Execute(Replace := wdReplaceAll) 
    else 
      WordApp.Selection.Find.Execute(Replace := wdReplaceOne); 
    { Сохраняем word } 
    WordApp.ActiveDocument.SaveAs(ADocument); 
    { Всё нормально } 
    Result := True; 
    { Закрываем document } 
    WordApp.ActiveDocument.Close(wdDoNotSaveChanges); 
  finally 
    { Закрываем Word } 
    WordApp.Quit; 
    WordApp := Unassigned; 
  end; 
end; 

Пример использования:

procedure TForm1.Button1Click(Sender: TObject); 
begin 
  WordStringReplace('C:\SomeStrangeDoc.doc','Маша ела кашу','Маша съела кашу :)',
                     [wrfReplaceAll]); 
end;