Page down in terminal
There are two answers to this question.

The first is globally useable. I'll assume you're on a narrow/laptop keyboard. In those instances, Fn+Down ▼ acts as Page Down, and just to expand that thought out entirely:


Fn+Up ▲ = Page Up
Fn+Left ◀ = Home
Fn+Right ▶ = End


This will not work properly in some applications. Those that come to mind are things like vim, less, screen, and other "full screen/ncurses menued" programs. The above keystrokes scroll the terminal window, but if the terminal window has not been paged, then scrolling won't accomplish anything.

The second half of the answer is one that I can't answer without more detail.

Example:  


In less, the spacebar or ctrl ^+f will page down one screen full.  
In vim, ctrl ^+f will page down one screen full.  
In screen, you will probably never have to Page Down, but if you scrolled back in copy mode, ctrl ^+f works there too.


If I haven't answered your question, you will need to elaborate on it, heavily.