Attach a PDF file to filter_email_attachments

I am looking to add a pdf file as an attachment to a form.
I have created the pdf from the form data and saved it into a folder on the server and ‘just’ need to add it to the email attachments.

I was informed I need to use filter_email_attachments but I am not getting anywhere when trying to add the file.

If anyone could provide me with some thoughts on the best way to add a pdf file to the form email that would be great.

The page I need help with: [log in to see the link]