PDF Reader Pro supports family sharing only when you download it from App Store.
Family Sharing makes it easy for you and up to five family members to share iTunes and App Store purchases.
Here is what you need when Setting Up Family Sharing:
- An Apple ID signed in to iCloud and iTunes
- iPhone, iPad, or iPod touch with iOS 8 and later, or Mac with OS X Yosemite and later
More about setting up:
How to Share App Store and iTunes Store purchases with Family Sharing