Respectable software that pays per download won't be easy to find. Pay per download is almost exclusively to the software being adware, or being bundled with various toolbars and adware. Actually respectable software almost always pays by sale and not by download.
The reason for this, is if the merchant makes their money off of sales and not adware, then paying per download isn't a good idea for them. Depending on the origin traffic and content on the page the file was downloaded from, it can impact their backend sale conversion rate by factors of over 100.
If you're interested in offers that pay per sale, feel free to PM me