How can I sell a PDF file from a non-Zen Cart web page, but use Zen Cart to download?

[kuroi]

Well it can be done. But the product still needs to exist in Zen Cart. If so you can construct a link that will add the product straight to the Zen Cart shopping cart, and put that link on your existing page's "buy now button".

However, this is messy and although I've had clients start like this, all those that survived ended up migrating the products into Zen Cart.

This usually comes about because somebody wants to market test a product without thinking the process right the way through, gotten somebody to build a static site (or in one of our cases a flash site) and suddenly realised that they need fulfilment too, so try to bolt it on in a hurry. Better to get it right from the outset.