An NZB file is basically a set of headers for attachments on Usenet, such as a set of RAR files.
To retrieve the content, you need access to a Usenet provider. Unless your Internet provider offers its own Usenet access, the only free one I am aware of is UsenetStorm, which fetches up to 200MB per NZB. I would imagine scanned/PDF Magazines to be just a few Megabytes, so it’s worth a try. Basically what you do click “Choose File” and give it your NZB file and click the Download button. If it’s under 200MB, it will then give you a link to what it has fetched.
If it’s considerably larger, you can try NewsDemon. The 10GB block is â‚¬1.44 (likely $2 in US.) To use NewsDemon or most other pay providers, you will need a Usenet reader, such as Grabit (I’m not sure if they’ve a Mac version.) Basically what you do with Grabit or any other binary Usenet reader is give it the Usenet login details from your provider and then double-click your NZB files one by one to import them. The Usenet reader will then fetch them. If you’ve used BitTorrent before (e.g. uTorrent), it’s a very similar process, apart from that the files download straight from the Usenet provider (rather than P2P) and nothing is shared out.