Could always install a free PDF printer, such as Bullzip. Then just use the same software you normally use for invoicing but use the PDF printer instead of your normal printer at the printing setup screen.
I use 'PDF Creator', which is a PDF printer. I can't remember where I downloaded it from but I wouldn't have paid for it. Unlike a lot of "free" PDF printers which put a watermark all over your invoice until you pay the full subscription, this one doesn't.
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