Our point of sale software gives a lot of flexibility for sending invoices, statements and reports by email. All are very quick and easy to produce. The main formats people use are Word and PDF to send documents.
There are some points you should be aware of in your selection.
If you are sending to someone else and you do not wish them to change it, then the best format is PDF. Some advantages are that you will have fewer problems in your the other person reading the document. Unlike Word, PDF looks the same on all computers. The PDF format is trusted, many for example will be reluctant to accept a word document because of the danger of virus being transported in Word. PDF is free to read and is widely available. The PDF gives you a professional look. It is also better legally.
Word and Excel, I would only use if I wanted the other party to have the ability to change the information. Sometimes your clients have a legitimate reason for wanting to change it so its good you can provide it. The problem here can be is that different versions of Word change the look of the document. This is one of the reasons why professional writers complain that Word is unstable. I have seen professional invoices in Word looking messy and unprofessional as the look changed.
As such unless someone specifies Word or Excel, I recommend you use PDF option in our software.