App
Problems

I've been monitoring the Google Classroom Community support pages and it appears that the problem is an ongoing issue with the app. Google is aware of the problem is working to get it resolved. I've also made the teachers aware of this problem.

I performed some tests, and I too was getting FILE NOT FOUND when attempting to open PDF attachments from teacher assignments. I uninstalled the app, and was able to use the Chrome Browser on the device to access Googele Classrooms without any issues. Using the browser instead of the app means you lose the ability to use the onscreen-editing features for writing/drawing on the PDF, as well as the ability to upload directly from the device's camera directly to a Google Classroom assignment. To get content into Google Classroom using the browser on the device, you first need to save the document to Google Drive, then you can link to that document to Google Classroom.

I'll work on formalizing these steps and will try to get a video of an example that will demonstrate these steps.

If possible, use the browser on a computer of Chromebook to access Google Classroom, as this should work fine. You can still use the iPhone / iPad to create content and upload to Google Drive - just use the computer to turn in assignments.