You could for example do:
pWnd->SendMessage(WM_COMMAND, MAKELONG(ID_FILE_PRINTPREVIEW, 1), NULL);

This will trick MFC into thinking that the user has pressed a key combination that triggers the print preview. The framework will then call OnPrintPreview.
Replace ID_FILE_PRINTPREVIEW with whatever ID the menu item in the file menu has.

Another option is to insert a public member into your view class. This member will then call OnPrintPreview from within the class.