Obviously you/developers care that it's a cool API that will replace the values. Will the mass market? Probably not. They want to know what they have to put in and what they will get out of it. Some visual reference to this would be good.
After all it's called template2pdf not template2API2pdf :)
I can see that currently you're thinking in the developer mindset but with some tweaks you could make it accessible for those that can't just send a hash. It could be awesome for LOTS of industries. Basically any industry that has sales reps & prepares quotes/invoices on the fly.
Maybe they just get a URL bookmark that displays an input form where they can enter their changing values & a PDF e-mailed to them. How great would that be to do mid-meeting on an iPad! A professional quote done before the end of the meeting. No going back to their office to prepare it! Sales reps would love that.
I almost missed that the words Template, API & PDF were links - I only found the output example by chance on second look! The output is super important, show it straight away!
Try to find a way of showing the template/input without it being a download.
Let people test it straight away - only ask for the e-mail/payment when they want to save their finished template.
As for not being a spammer, you can easily create content that isn't 100% sales pitch. So maybe you start a blog about automating & streamlining procedures, admin hacks etc - then at the end of each post you can refer back to how great template2pdf is.
Also, you could get a designer to create some pretty sweet template designs & make them part of the paid tiers.