After you've edited your assignment using Lumin PDF, you can easily sync it back to your Drive from Lumin to hand into your Google Classroom.
1. Sync your completed assignment to Google Drive from Lumin PDF. Click the Google Drive logo above the toolbar
2. Once successfully synced, go back to Google Classroom from your Google Account
3. Select your Class, and then 'Classwork' tab
5. Click on the assignment you want to hand in and 'view assignment'
6. Select 'Turn in'