The first bit may not be much help as I form invoices by using mailmerge to a database.
However, the following might help: If you download a free program such as Primopdf, you can have an invoice open and convert it to pdf. via the print option. You can then attach it to email. Pdf is pretty much a universal format across all operating systems (it means Portable Document Format I think). When you install Primopdf, it adds to your "print" options and shows up as a printer so that you can "print" to file. If you take this route, refuse any optional extras the installation offers - just in case it tries to change browser settings etc.