Well, I am not sure what Inbuilt Apps in the S3 and S6 are that are supposed to do Annotation of PDF's, but Xodo was also installed, and does seem to be reasonably effective, so I will use that.
Thanks for the assistance.
Sorry, I didn't have time last night to look at the video you linked and I have been away all day for work. Lucky me essential service duties today so got out and about.
I think you're correct about how the S3 does the annotation. I didn't realise exactly what you're after. I think the S Pen and the like of Samsung Notes will do some of what you want. I haven't investigated Xodo though I do seem to recall looking at it for something but never ended up using it. Sounds like that's more like what you want.