Step 1-  Go to uploaded documents in an activity

Image: Document cards inside an activity 

Step 2 - Click to view the document 

Image: Document view with the comments panel 

Step 3 - Add a comment and tag your teammate if needed

Step 4 - Getting notified about tags on documents 

If someone tags you in a document, you will get a Todo on your dashboard, here you can keep a track of all the documents you were given feedback on. You can simply click on the Todo to visit the document you were tagged in.