Yes, you can add a PDF attachment while creating an invoice. To add an attachment, you need to follow the below steps.

1. Click on the '+' sign to add an invoice
2. Below the amount due section, there is an 'add attachment' option.
3. Clicking on the attachment option, you need to select the file that you need to add.
4. Click open to add an attachment. 
5. Click on save for creating an invoice with an attachment.