If your student has turned in an assignment in Google Classroom, but you are either unable to view or edit the document in DocHub, your students should set you as a Contributor before (or after) turning it into Google Classroom. This way, you'll be able to access it and grade it using markup in DocHub.
Alternatively, you could just make a copy of it which will keep the student's work and let you add your own grading annotations in a new copy. To do this, just click on File menu > Actions > Make a Copy. Here is the screenshot:
Here is how your students can add you as a Contributor:
- Open the document that needs to be shared
- Click on the Share or Send button
- Enter your email address into the bottom field as a Contributor and click Add
- Close the Share window
- Save it to Google Classroom and turn it in