My used 2007 G35 Sport Sedan didn't have the owners manual.
Is there an online copy that I can download for free without a catch?

Google is very friendly

http://www.infiniti-club.org/manual/12.pdf

If you want the Factory Service Manual it's also available on the Nicoclub website, free to everyone, no account needed, no bs.

I strongly recommend you download the ENTIRE document for your exact vehicle if you do your own maintenance work it's an incredibly valuable resource and far better than a Haynes/Chilton. Last year Nissan North America was trying to flex their D and made Nico pull down all the FSM's. Fortunately they went back up but there was a couple months without them available. They also recently redesigned the web page for them and the G35 FSM's weren't available for a month, thus why I recommend downloading a copy onto your pc or phone.

Everything starts at the FWD-Forward document.

https://www.nicoclub.com/infiniti-service-manuals