Automatically scroll is greyed out

Hello

I don't use Adobe a lot, so please excuse ignorance. If I try to initiate auto scroll it won't let me. I can't click on the mouse wheel, Ctrl+Shift+H doesn't work, and View-Page Display-Automatically scroll is greyed out. I can auto scroll in every other application, so what am I missing?

I'm using a four month old pc, Reader was downloaded in the last two months. The pdfs are sent to me from book publishers for me to comment on. Any pointers, please?

Thanks very much.