When you purchase PDF Expert on the App Store, you get the full version with PRO functionality right away. When you purchase it on our website, you can either download the app for free to explore advanced PDF features before purchasing or press the "Buy now" button if you are ready to make the purchase.
If you are not sure which option to choose, please check out this article.
How to upgrade PDF Expert for iOS to PRO:
The payment will be charged to your iTunes account. The subscription renews automatically until you cancel it.
You can cancel a subscription anytime in the Settings of your device at least one day before each billing date.