Install the Kami extension for Chrome.
Check out free Kami webinars.
Kami is a Chrome Extension that allows you to annotate on a PDF from your Google Drive. When you click to open the PDF, you will have the option to open with Kami. Doing so will allow you to use the annotation tools.
You can also annotate in Microsoft Word or Powerpoint. This is probably one of the most common ways to annotate for teachers making explainer videos, as it gives you a "blank slate" to write on. Click on the image for more specific directions but basically, open Word and select the "Review" tab at the top. Then simply click on "start inking".
If the work/problem that you are wanting to explain/solve originated in a Google Doc or a PDF, here is a trick that many teachers will use. Use the Snipping Tool already on your device and take a snip of the problem/question. Copy and paste the "snip" image into a Word Document. Now you can show the problem/question as the students see it and use the whole page to write.