If you are permitted to submit your invoice via email then it is useful to convert the invoice into PDF format so that it cannot be altered. There is a free PDF creator which you can use at PDFCreator. This creates a PDF file from any application which can then be emailed to the client. Better off, if you can use a software system, it will automatically generate invoice in PFD format, you can also email it straight away from within the a software program.
About proforma invoice _ A Proforma invoice is an invoice provided by a supplier in advance of providing the goods or service. A quotation in the form of an invoice prepared by the seller that details items which would appear on a commercial invoice if an order results. It is more of a customs declaration form used in international trade that describes the parties involved in the shipping transaction, the goods being transported, and the value of the goods. It is the primary document to declare value for customs. It is not a true invoice, because the seller does not record a pro forma invoice as an accounts receivable and the buyer does not record a pro forma invoice as an accounts payable.