I have an advertising agency and I want to write a professional letter to introduce my company to a new client working on farmacutical career

I suggest speaking to a librarian about a book on letter composition. Also spelling is extremely important, so buy a dictionary. You have 2 serious errors in your question here. If you are not near a library, try a bookstore, or do online searches.
I would say that the first rule of a letter of introduction is to keep it short, less than a full page. Start with a one sentence paragraph stating the purpose and end with a one sentence paragraph thanking them for their consideration. Fill the middle with concise facts, followed by a little subjective opinion of your own.