This plugin automatically adds a sticky ‘Go to Top’ button to posts and pages.
(Scroll down and watch the lower right corner of this page, to see how it works!)

It’s a simple, configurable wrapper for Mark Goodyear’s ‘scrollUp’ jQuery plugin:
https://github.com/markgoodyear/scrollup

Download and install it from:
https://wordpress.org/plugins/float-to-top-button/

Hiding the button on specific posts and pages

To disable the button on a specific post / page, add a custom field named hide_fttb and give it the value Y.