If you want to use PureVPN with built in Unifi VPN Client, you have to simply edit the .opvn file from PureVPN.
The original .ovpn has some hidden attributes that has to be removed with Notepad++.
When its done you can simply use it with Unifi devices.

Tutorial:
1) open the .ovpn file in Notepad++
2) Show all characters. There is an icon in the toolbar or go to the menu View-Show Symbols-Show All Characters.
3) Go to Search -Replace
4) In the field "Find what:" enter: \r\n In the field "Replace with:" enter: \n
5) Select Search mode = Extended.
6) Click Replace All and save the File
7) Done