Symptoms
When an invoice is generated and sent automatically from a Recurring Invoice, there is no invoice PDF attachment under the Notes & Attachments section.
Findings
This is expected system behavior.
When an invoice is sent automatically from a Recurring Invoice, a PDF is not attached to the invoice record.
A PDF is attached only when an invoice is sent manually using the invoice's Email / Print Invoice PDF button, provided the After sending the email, attach PDF to the invoice checkbox is selected.
A PDF can also be attached by clicking Attach PDF to Invoice without sending the invoice.
Summary
The absence of a PDF under Notes & Attachments for automatically sent recurring invoices does not indicate an issue. It is the expected behavior for invoices generated and sent through the recurring invoice process.