Shortcut key for scrolling inside a terminal

From the tmux FAQ:

******************************************************************************
* PLEASE NOTE: most display problems are due to incorrect TERM! Before       *
* reporting problems make SURE that TERM settings are correct inside and     *
* outside tmux.                                                              *
*                                                                            *
* Inside tmux TERM must be "screen" or similar (such as "screen-256color").  *
* Don't bother reporting problems where it isn't!                            *
*                                                                            *
* Outside, it must match your terminal: particularly, use "rxvt" for rxvt    *
* and derivatives.                                                           *
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http://tmux.git.sourceforge.net/git/gitweb.cgi?p=tmux/tmux;a=blob;f=FAQ

Try Shift+Page down or something similar (eg. Ctrl+Alt+Page down).

Page down sends the page down key to the program running inside the terminal. If that's your shell (bash/zsh/whatever), it probably doesn't accept the key combination, and ends up trying to parse the encoded key, which contains ~.

It's more likely you were scrolling in your terminal program itself, using the scroll bar or something similar, so you want a key that your terminal program accepts directly. You didn't specify which program you're using, but Shift+Page down is common. You can also try different modifier key combinations; your terminal's key binding setup might also have more information.