Renaming the PDF when invoice exports to email

SOLVED

I am trying to rename the PDF at source when exporting an invoice to email.

Current name when exporting is; Company Name - Invoice No. 1234

I am just looking to remove one character due to the platform that our biggest customer not liking the format, and we are having to manually rename files at the moment which seems a waste of time! 

Would like it to be; Company Name - Invoice No 1234

I've managed to rename the subject, but it's just the pdf name.