On a mobile device, there are a few workarounds you can try in order to add a signature line to your documents in Word. You can use the underline key or a single cell table to add a signature line to your Word documents. You can also insert a graphic of an actual signature. However, you can’t add a digital signature line to Word or Excel files from your mobile device without installing an add-in.
To add a signature line, use the return key to position the cursor where you want the line to begin. Locate the underscore key on your keyboard, and then hold the key down until the line is sufficiently long.
On the Table tab, tap Delete.
Delete rows and columns, leaving only the first cell in your table.
On the Table tab, tap Table Styles.
Select a style that has a gridline under the header row, but has no other gridlines.