When viewing your PDF document, click in the middle of the screen to bring up in the menu.
Click the "comment bubble/pencil" icon (3rd from right) to view the annotation tools. Click the "signature pen" icon (2nd from right).
Click on your document to bring up a contextual menu. Click Add Signature. You'll be in the "Sign Here" mode where you can create a signature with your finger. When finished, click Save. (If you later want to edit, click Edit Signature in the contextual menu. You can clear the one you created, and create a new one.)