Considering your traffic is built around a very specific task (file conversion), and it is inherently people who want something for free, I would approach this by contacting some of the biggest companies of the niche who make similar products and ask whether they would be interested in some kind of cooperation.
For example, every time someone converts a file, you thank them and then show them a link that in case they need to also edit the file a bit, they can use this product.
I could imagine developers of such product would be very keen on having their product recommended this way.