View-scroll-half-page-forward is an interactive compiled Lisp function in `view.el'.

(View-scroll-half-page-forward &optional LINES)

Scroll forward a "half page" (or prefix LINES) lines in View mode.
If LINES is not omitted, the "half page size" is set to the minimum of
window height and the absolute value of LINES.
LINES=0 resets "half page size" to half window height.