It has disappeared from the Google Play Store.
I don’t know when this happened.
I do know the app was waaaaaay overdue for an update.
I guess the Chinese developers went out of business.
It’s too damned bad another PDF app developer didn’t see the wisdom of buying the source code and incorporating it into their own app.
SmartQ PDF Reader was the fastest damn PDF reader I’ve ever tried for Android.
This is a huge loss for reading Google Books PDFs on Android tablets — and possible even under Android emulation on Chromebooks.
The best alternative now is iBooks on iOS — until GoodReader PDF for iOS is revived (if ever!).