So far, the ability to add comments to documents stored in SharePoint Online (SPO) and OneDrive For Business (ODFB) has been restricted to Office documents. Fortunately, this restriction has been removed and we can also add comments to non-office files through the common file viewer in Office 365:
Any comment added to a file gets registered as part of the file activity. Other users will be ablle to addd new comments to the previous comments, so notifications will be sent to the authors of the initial comments. Something that is not supported for now is the ability to mention other user in a comment in a file.