Using 16.3 currently. For some reason it is not a default app to open pdfs (say from Downloaded folder). My tablet says it can't open a file (i have no other pdf viewer installed, even google one is removed). I see no option to make Adobe Reader a default.
I believe for this you need to change the default app settings in your Android phone.
For this please try and see if the steps help:
navigate to Settings>Apps>All and then select the App currently associated and remove it as default and tap clear defaults.
By doing so now when you open your file you will be prompted to choose a default App again.
Let me know if it works or you need more help.
After some testing i have actually found out that this only happens with some pdfs, and other pdfs open automatically with Adobe Reader. So it is probably some problem with those pdfs or Android not liking something in their names.