How alter page content before printing?
The print friendly module uses the theme system to output your print friendly page. So you need to do this on theme layer using a preprocess function. Use THEMNAME_preprocess_page or THEMENAME_preprocess_print in your template.php. Check the theme_hook_suggestions for more options. From the module 

 $variables['theme_hook_suggestions'] = array();
 187   $variables['theme_hook_suggestions'][] = "print__node__{$type}";
 188   $variables['theme_hook_suggestions'][] = "print__node__{$type}__{$nid}";
 189   $variables['theme_hook_suggestions'][] = "print__{$format}";
 190   $variables['theme_hook_suggestions'][] = "print__{$format}__node__{$type}";
 191   $variables['theme_hook_suggestions'][] = "print__{$format}__node__{$type}__{$nid}";