Create, edit, and sync-with-PC txt file on Android
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.