Copy documents to a subsites

I have a site collection 'MySiteCollection' which constains many subsites like 'SiteA', 'SiteB', 'SiteC' ...

Each subsite contains it own subsites. For example 'SiteA-1','SiteA-2','SiteA-3'.

And i want to copy a document from a library in the rootsite of 'MySiteCollection' to a subsites like 'SiteA-3' with REST api.

I found some example, but they don't work for me. So if you have a solution for this problem, i would be glad if you can post it.

Thank you very much.