In Moodle as teacher, create an assignment with file submission. Install the app in an iOS device. Login in the site as a student. Share a file with the app. E.g. you can just search "Sample pdf" in google and share any of the PDFs with the app. Back in the app, go to the assignment you created in the step 1 and click to add a submission. Click to add a new file and select "Shared files". Check that you can select the file you shared and it is added to the submission.