Hi,
This error indeed means that you're using an old version of the plugin.
You can see the changelog of the plugin here:
www.hikashop.com/marketplace/product/18-plugin-pdf.html
As you can see, we regularly release new versions of the plugin and the support for Joomla 4, which transitioned from isAdmin to isClient was done with the 1.0.26 in 2019, so this means your version is older than this.
Old versions also don't support the update notification system of Joomla. This was added only about 1 or 2 years ago so it's normal you are not notified of the update in your backend. Once you update to the latest version, you'll be notified going forward.
Regarding whether you need to pay for updates, here are the details:
Following your purchase of a plugin on our marketplace, you get access to updates of the plugin for 4 years for free. You can just use the same download link to get the latest version.
After the 4 years period, you need to purchase the plugin again in order to get a new donwload link which will also be valid for 4 years.
Since your version of the plugin is from 2019 or before, you need to purchase it again I'm afraid.