Hi and thanks all of you for your input. The problem has been resolved. What actually happened was that I could see the attachment in the list view only. When I opened a note, there was no file attached first, then after some time I could see the file in the note itself, but could not open it. I have WPS Office and Hancom Viewer, so there should be no problem. Then the issue somehow was resolved on its own. Now everything works fine.
Another question is a bit off topic but still... When I tap the attachment to open it, I have only two options for .doc files to choose: WPS Office and Hanco