If you are using the email module out of the appstore, you have to create a an Attachment document, which is part of the module, relate is to the email and you can use the CopyAttachmentContent function from the community commons module to copy your invoice the the new FileDocument.
Create an object EmailTemplate.Attachment, copy the pdf into that (java-action DuplicateFileDocument) and link it to the mail object you created.
If you don't like the objects to be duplicated create an assocation from pdfdocument to email and change the mf IVK_SendMail to retrieve the pdfs directly