There is an add-in to Google Drive that allows you to edit PDFs. The programs accesses the PDF from Google Drive, opens it, and then saves the edited PDF back to your Google Drive, where it can then be uploaded to Google Classroom.
The add-in is called Kami. Click here for a quick tutorial showing how to use Kami with Google Classroom.
We have negotiated with Kami to provide their full license free-of-charge until Apr 30 for all users with a dallaschristian.com email address.
If you're using the Google Classroom app on your mobile devices, editing / annotating many file types is built right in.
https://support.google.com/edu/classroom/answer/7080036?hl=en
A video example can be seen here.