Using the online reader, locate to a page you wish to share. Click on the chain (link) icon on the top right corner to obtain a unique link. The link obtained above can then be shared on Google Classroom.
Alternatively, you can click on the Google Classroom icon (green icon with a white person) to automatically populate the content and link.
Choose the class you wish to share the page to and the action (whether it is an assignment, question, announcement) and press "Go"
Fill in the remaining details as necessary (i.e. due date), the content will be automatically populated and ready to post.