How alter page content before printing? In my project, I am using "Printer-friendly pages" module in order to add print button to my project.

for example when a user is in "checkout/5/review" page, print module create a page with "print/checkout/5/review" address.

this process works fine but I want to alter the created page for printing "print/checkout/5/review"

I tested hook_page_alter but it doesn't work.
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}";