Help using the PgDn key...

I tend to use this key to scroll through longer documents and chapters, but I can’t find an option to make this consistent within different projects.

In one project hitting this key scrolls the text one window’s length.
In another project, created a week later, the key takes me to the end of the chapter, making navigation within the text pretty much impossible.

I’ve searched through the options and the manual, and I can’t find the setting to adjust the use the key - what am I missing?