Strange Saving Behaviour of Samsung's "Write on PDF"

I did some work yesterday using Write on PDF on my Samsung S6 Lite tablet. Come this morning, and it didn't save. Bit of a nightmare. Anyway, I redid the work, but decided this afternoon to test out exactly how the saving works. It behaves very strangely! I have uploaded a video (screen capture) to YouTube showing exactly the behaviour: https://youtu.be/ksfVwWm6iUY . Basically, when I open with Write on PDF, it doesn't seem to save successfully. Instead, I need download it, open it up and write on it before saving. Even then, it saves some random stuff. The video hopefully makes this clear. >I want to open a PDF from a webpage (or app) in Write on PDF, do some writing on it, then save. >How do I get this to _actually save_?