請問誰人有 Casio JS - 120 計算機的說明書 ?

I'm not sure if the PDF manual of this model is downloadable on the web. Probably not from Casio's website since its support is mainly on educational tools (scientific calculators etc.)
I suggest you contact Onflo and/or Multi-M to see if they can provide you with a copy, or additional access to a PDF version.
http://www.onflo.com.hk
http://www.multim-group.com/