Our billing company prefers the invoices printed in PDF format. We send them daily batches via secureFTP for them to process. There is a really good program that their IT person recommended that I will share here. This program creates PDF documents simply by choosing a printer in MSWindows print dialog. Very easy to use and it is free!
http://www.cutepdf.com/Products/CutePDF/writer.aspYou will need to download both the GPL Ghostscript 8.15 (5.01MB), a convertor and the CutePDF free download (1.54MB)