https://awwapp.com free version has some limitations, among others, no save, just export to PDF with watermark and no multipage, and a bit disturbing add banner at the bottom

BigBlueButton offers shared board. You can either draw to the uploaded PDF presentation or used the default one - go to next page. All-drawing can be anabled by using the menu on right.

https://whiteboardfox.com/ is free by default, very simplistic, but perhaps good enough for start

https://miro.com/online-whiteboard/ 3 active boards for free

overview https://zapier.com/blog/best-online-whiteboard/