page break when printing only

This can be managed using the CSS page-break-after Property

Ok, I ad an empty span with a css class, then:
.page-break {
page-break-after:always;
}

Now I have to figure out how to print a foot note on the bottom of each page !

Is there a way to set custom style in a file, and not in the wordpress/settings/print-o-matic ?

With Print-O-Matic you can either add the css to the settings page, or the style.css file of your theme (hopefully child-theme).

Print-Pro-Matic allows you to load an external print_only css file using the css_url attribute like so:

[print-me css_url="https://example.com/css/printmonkey.css"]