Many of our students are using iPads for instruction both at home and at school. There is a way to have students "mark up" documents either by using an app called PDF Cabinet if the teacher is using Schoology, or by the built in features of Google Classroom. Giovanna has made two videos to assist with this process. Annotating PDFs in Google Classroom as assignments and PDF Cabinet App-Demonstration Video
Here is a step by step document on using PDF Cabinet App.