Inserting a happy face in the messages is so popular that you cannot even realize when you do that. Inserting a happy face in a word document has the same rules to add a happy (sad, neutral) face in the messages.
To add a happy face in the word file, you need to type the symbols and make them horizontally looks like a happy face: two eyes (a colon), a nose (dish, optional), a happy month (right side parentheses);
To add a sad face in the word file, you need to type the symbols and make them horizontally looks like a sad face: two eyes (a colon), a nose (dish, optional), a sad month (left side parentheses);
To add a neutral face in the word file, you need to type the symbols and make them horizontally looks like a neutral face: two eyes (a colon), a nose (dish, optional), a neutral month (broken bar).
Alternatively, you can add a happy (sad, neutral) face with the steps below, or check how to add the full set of Emoji:
Step 1: Click the "Insert" tab from the ribbon;
Step 2: click "Symbol" in the "Symbols" section, then click "More symbols" from the drop-down list;
Step 3: Select "Wingdings" in the "Font" box, and you will see the happy (sad, neutral) faces in the table;
Step 4: Click on the happy face and click "Insert" at the bottom, the happy face will appear in the word;
Step 5: Click "Close" when you finish all the faces.