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.
As long you are looking for something online, you have to be prepared to face some hurdles. There are numerous hurdles that you will encounter and the top on the list are scams. Although you may find an invoice format or template that you think is good, you may be made to pay a lot of money for it. It is thus important to look for trustworthy sites that will not use up a huge chunk of the company funds. One such site is the Microsoft site. The site offers businesses different templates for all the forms and documents needed to keep a company running. It is also the best place to find an invoice example, which will look perfect and above all official and professional.