I use Notepad++ v7.8.5 on Ubuntu 18.04. I run Ubuntu and Windows in dual boot mode.
I was not able to open text files in Windows (automatically mounted onto Ubuntu as /dev/sda3). Below is what I found to be useful.
Open UbuntuSoftware > Find Notepad-Plus-Plus (WINE) > Click on it > Click on Permissions > Find “Read/Write files on removable storage devices” > turn it ON.