What you are offering is NOT a free tool -- it is a demo, for a tool for which you are charging $12/month. No reasonable person would interpret a grand total of 3 exports as enough to justify calling this a "free" tool.
This is to say nothing of your violation of AGPL on the use of MuPDF, which has been pointed out here and elsewhere.
But of course, you're free to Show HN a paid product; just kindly don't insult our collective intelligences in the process.
Commonly known as a "free trial"
Imagine a pizza place where you can try three slices for free before you order one?
Or that you can make a pizza at the shop, add and remove topping as you wish until you're satisfied?
A better comparison would be a pizza shop at the end of a long hike that advertised itself online to offer infinite amount of free pizza. So you go on the hike and then it turns out you only get one slice and have to pay a fortune for the rest. You planned to get free food st the end of the hike, but it turns out the food you eventually will have to eat is not free and not even cheap.
This is not free, it is s free trial.