I have been getting a ton of questions about making PDFs editable so people can write on them. I have a ton of suggestions but please remember, kids are using a ton of different devices. you can edit a PDF in Google classroom on an iPad that was made in the last 3 years, but you can't naturally edit a PDF on a computer/Chromebook in Google Classroom without everyone having to sign up for extensions and add ons.
1) My first suggestion is to just upload the PDFs to your Google Classroom and not digitize them for students to mark up. In my opinion, completing math using digital tools is cumbersome (my husband is a middle school math teacher and is full time teaching online so I have seen him work through different variations.) My first suggestion is to post it as a PDF and prompt the kids to either print it (which most students don't have a printer) or just keep the worksheet open on the device and complete the work on another piece of paper, just as if they had a textbook they couldn't write in. My son's 7th grade math teacher did this and then asked them to take a picture of their work and email it to him/upload it to Google Classroom. It wasn't any harder than working out of a textbook.
2) If you do want to digitize, here is a way to turn any PDF into an activity in Google Slides that students can mark up with all the tools in Slides: https://drive.google.com/open?id=1O-siUU2jxz3q0ZycixELNU0e8d0M22Lq
3) I love the website Edulastic! It is an amazing place to make digital assessments/activities but especially math. You can build an assignment from other people's assessments, access a standards based library to add questions, but also upload a PDF and turn any PDF into a digital assignment and the best part is it naturally comes with math tools!!! https://www.youtube.com/watch?v=uOelJAEiYoU The best part about Edulastic is it will "grade" the assignment and keep the data for you!
4) There is a fabulous annotation tool called Kami. You can go here to sign up for a free account till this is all over: https://blog.kamiapp.com/remote-learning-with-kami-during-school-shutdowns/
Kami allows you to annotate PDFs with all the bells and whistles of Adobe and even a bit more. You can integrate it with your Google Drive. When you push out an assignment, students are prompted to create an account. This takes a bunch of clicking and accepting permissions so I'm a bit cautious about using it with kids without being able to walk them through the process. Also, it is made for computers and Chromebooks so there are "workarounds" to get it to work on phones and tablets (https://help.kamiapp.com/en/articles/3056987-device-compatibility-for-kami). Here is Kami in action! https://www.youtube.com/watch?v=oTDA0dUsYvg
5) If you are using Seesaw, you can take a picture of any PDF and create an activity so students can write on it and even complete some simple drag and drop activities.
I know this is a ton of information but I wanted to give you an overview of what you can do and what is easiest for the students.
