Proforma invoices basically contain much of the same information as the formal quotation, and in many cases can be used in place of one. It should give the buyer as much information about the order as possible so arrangements can be made efficiently. The invoices inform the buyer and the appropriate import government authorities details of the future shipment; changes should not be made without the buyer's consent.
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.