When someone completes a purchase of a printed book, I'd like to offer them the PDF of the title free of charge. Is there a way to do this easily within a print project?
No, not within one project.
You must create a new Ebook project, select PDF, set it to Direct Access and provide the link only to those whom you'd like to have the PDF free.
You could create a free PDF eBook version and set it to direct access.
In your thank you note, include the URL for the PDF version - or a link to your website where they can download the PDF.
Keep in mind that there is only one thank-you note that will be sent to everyone who buys any of your projects.
I'm full of ideas today....
Another option would be to include the URL for the direct access, free PDF in the front matter of the book. That way you can provide a specific address for the PDF to someone who you know bought the book.