You can press C-h k C-PgDn and learn that scroll-left accepts an
optional argument that specifies how much to scroll, and that the
default is window width minus two. Then, you have options.
Firstly, you can specify this argument each time you use scroll-left:
C-u 20 C-PgDn will scroll by 20 columns.
Alternatively, you can write your own version with a different default
and bind it to C-PgDn:
(defun my-scroll-left (&optional arg set-minimum)
(interactive "P")
(scroll-left (or arg 4) set-minimum))
(global-set-key (kbd "C-<next>") 'my-scroll-left)