Take a look at the first two extras here if you haven’t already. They may do what you want, and if not, it might save you time to modify one or both of them.
If you are using FormIt anyway, you could maybe make use of the formitsaveform hook and use that data or count of submitted forms, etc. I don’t think it delivers that by default but maybe with some tweaking this could be a useful start. Just an idea though.
If you are tracking downloads vs Ad Performance, should you not use tracking code from the Ad serving site, eg. If you are using AdWords, use their tracking code or use Google Tag Manager/Google Analytics to track the number of Downloads, and link that to the Ad. This way you can link multiple Ads to the same PDF and track which one converts the most, rather than creating multiple forms/static reseources to track the ad.