Answer
It depends on whether the attachment content is constant or record-specific. For constant documents that should be attached the same way to every instance of the report (like a fixed Terms and Conditions PDF), Report Rules can handle this via the standard PDF-insert pattern. But for genuinely invoice-specific attachments — actual DocMan documents that vary per individual invoice record — achieving a single combined PDF isn't something Report Rules can do through configuration alone, since Report Rules work with a fixed, predetermined file rather than dynamically pulling in whatever DocMan attachments happen to exist for that specific record; that scenario requires a genuine customization instead.