According to at least Reddit, many of the users for this program are college students (I amongst them). Given the starving nature of the general college student, subscription plans are a no-go.
Suggestion: A specific one time payment of 5 dollars or so for the better PDF merge UI and maybe some other UI conveniences I'm unaware of. With the additional benefit of removing the local 'ads' for upgrading to pro.
People that use the product and love it WILL end up upgrading as it is needed in the future. The company isn't going to lose out on prospective hypothetical purchasers when they already have consistent users that strictly use the freemium version; the most effective advertising you have is the app itself. The intrusive ads are well established and are not effective motivators when price tags are high for the poorer demographic of users. Consider the $5 upgrade a simple token of appreciation for the wonderful app and let the poorer folks go free.
I just came here to say this anywhere I could. I love Xodo, I use it on 3 devices but I'd never pay 150 bucks a year for bonus features. That's insane, make a lower price point for individual users and you will make more money.