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

1. Click on the '+' sign to add a proforma invoice
2. Below the terms and conditions, there is an 'add attachment' option.
3. Clicking on the attachment option, you need to select the files you need to add.
4. Click open to add attachments to the existing invoice. 
5. Click on save for creating a proforma invoice with an attachment. 

Note: You can add only PDF & image files  as an attachment.