I think this is a paid account thing. Someone correct me if I'm wrong.
I signed up for the 14 day trial and it has the option to send the classroom the assignment via Kami. However, it won't automatically open in Kami. The students would still need to open it, click on the three dots on the right to open in a new window, and then the option to Open in Kami would show up above the document.
Instead, I recommend you assign the PDF like you normally would using Google Classroom, setting the option to make duplicate copies for all students. Have the students go to Google Drive INSTEAD of Google Classroom. Have the students set their settings to have PDFs default to Kami. Then have the students find the Google Classroom folder in their drive. Favorite the folder as 'Math Folder' or 'Reading Folder'. Have the students view files in list view because the titles are too long to read. Typically the newest assignment is on top in that folder.
The big plus with a paid account that I have noticed is that there is an option in their Kami viewer to Turn In to the classroom, so they don't have to go to the Class page to do that. It saves them some steps there. It does nothing for making it easier to open it directly into Kami.