Great new feature in Google Drive Comment on a PDF or Microsoft File
One of the amazing features in Google Drive is the ability to comment within a Google file (Docs, Sheets, Slides, etc). However, you could not comment within a file such as a PDF or a Microsoft file - but now you can!
When you open your file (my example below is a PDF) in the upper right you will see the actions button, download, print, and now the comment button.
Embedded below is a GIF followed by a 35 second YouTube tutorial.