How can i open next form by pressing a specific key (e.g -if i press "PgDn key" then form5 will be open)

I assume it needs to be done when this key is pressed in some form. Override the method System.Windows.Forms.Form.OnKeyDown and use event arguments parameter passed to this method to determine what key is pressed. Here is the problem: usually some form's child control is focused; and it grabs keyboard focus out of the form itself. Activated form rarely has a focus like a control. So, you need to preview events by the form. To do that, set the property System.Windows.Forms.Form.KeyPreview to true.

See:
http://msdn.microsoft.com/en-us/library/system.windows.forms.form.keypreview.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.forms.form.aspx[^].

Now, keep in mind that if you use this exact key you can greatly confuse your user. PgDown should be reserved for "Page down". Better pick up some more reasonable key or key combination — there are many of them on the keyboard. :-)

—SA