Create, edit, and sync-with-PC txt file on Android There are many apps that allow us to take notes on android. 

But just like this thread (Is it possible to create a plain text file in android?), I want to create and edit plain txt file on android.

But moreover, I also want to automatically sync with a txt file in my PC. Ideally, dropbox. So ideally, I edit txt file on my PC then the edit will sync to my Android phone, and vice versa.

But if this ideal solution (dropbox solution) is not available, then I am fine with less ideal solution which will sync txt with PC.
Give Standard Notes a try. Works on multi platform, syncs across all platforms, permits tagging, encrypted, and open source. Don't be discouraged by Beta in the description. It's perfectly stable and I have been using it for nearly two years.

Pro version (expensive) does much more. Sync to Google Drive (also Dropbox if I am not mistaken is a pro feature) but you may not really want that once you see the way app does things.