I presume the app was deployed via the Volume Purchase Program in some form. If so, it has not been possible to deploy in-app purchases through VPP.
I did some reading about PDF Expert itself. Apparently now it is a subscription based app...you get it for free from VPP or the App Store, but then there is some method to activate your subscription. I would reach out to PDF Expert support if you have a subscription and find out what their method is for mass deployment.
We run a script to automatically delete this trash program. It's adware (Install Core adware group) and you should probably get rid of it too.