Yes, on the Documents page, select "Engagement Letter" in the dropdown and click "Browse" to select the pdf on your file system. It will be added to the list below and eventually you will be taken to DocuSign to place the signature coordinates.

(For detailed instructions, go to: https://proconnect.intuit.com/community/manage-integrations/help/how-to-use-esignature-in-proseries-.... 
Expand the section titled "Sending a Request with additional documents")