Attach to email a PDF generated by FPDF script

I’m using FPDF to generate PDFs from data and it’s working perfectly. I now want to send a generated PDF as an attachment in an email.

What’s the method for doing this? The PDF is an invoice so I’ve tried putting this in the attachment:

/invoice.php?output=email&inv={{InvoiceID}}

but it’s giving me this 500 error:

message: "fopen(/home/public_html/invoice.php?output=email&inv=5): failed to open stream: No such file or directory"

The PHP script is definitely in that location so I’m thinking I’m going about this the wrong way.

Is anyone able to offer some help?