you will not be able to achieve that via workflow if you are putting the file in communication subtab -> Files
there are two approaches, use a document field on the body and upload the pdf there then it will be easily sendable via workflow
another approach will be a short cut and time consuming, if the user stick to attach file under coomunication subtab -> files, then you will have to go via suitescript, with suitescript search for the file attach to invoice, there can be multiple files (how u will identify? if u can use OCR then it will be easy otherwise give standard prefixes to every file upload and filter the files with that unique prefix such as document number etc), once you find use N/email library to send that file via email
Let me know if u have questions