Set pagination posts per page

snippet.php

// functions.php
function custom_posts_per_page($query) {
    if (is_home()) {
        $query->set('posts_per_page', 8);
    }
    if (is_search()) {
        $query->set('posts_per_page', -1);
    }
    if (is_archive()) {
        $query->set('posts_per_page', 3);
    } //endif
} //function
add_action('pre_get_posts', 'custom_posts_per_page');