Synonyms multi word search issue

I have created an index with synonyms.txt file in which there is a word GDM and corresponding synonym is General Data Manager and the expectation is If I want to search "GDM" it should show results which has "GDM" or "General Data Manager".
But the search results is not what I want. It shows documents which have "General", "Data", "Manager" or "GDM". I just want to get results which has "GDM" or "General Data Manager" in the documents.
So, what can I need to do? Thanks.