To attach a file to the custom email, simply click the "Add Attachment" button.
After you click the button you will be presented with a normal file selection, and you can choose the file you would like to attach.
There are two kinds of PDFs you can attach to custom emails, the first is a default summary that is available on all plans, and cannot be modified. The second kind is Paperform's Custom PDFs, which can be created under After Submission → Custom PDFs on Pro and Agency plans.
To attach either kind of PDF to an email you can simply select them from the "Attach Custom PDFs" dropdown in the custom email setup.