Make your own on word or download open office, it's free and just as good.
You can have different files for different companies if you have regular ones.
Set up a cleaning folder with your word/open office invoice files then change when needed and instead of saving as, export as a pdf file and you can also save the original word file too. Put your pdf invoice in a folder called un-paid and email them with it and when they pay move it to a folder called paid, simples, works great for me.