Customize posts-per-page based on some condition.

wp-customer-posts-per-page.php

<?php

// add filter for front-end requests
if (!is_admin() && !is_feed()) {
	add_filter('parse_request', 'my_add_posts_per_page');
}

// add our condition-checking filter
function my_add_posts_per_page() {
	add_filter('pre_get_posts', 'my_posts_per_page');
}

// customize the posts_per_page, 
function my_posts_per_page($query) {
	// we only want our filter to run on the initial query, so we remove it here
	remove_filter('pre_get_posts', 'my_posts_per_page');

	// set your condition - can be whatever you want
	if (is_category('example')) {
		// set the number of posts per page
		$query->set('posts_per_page', 100);
	}
}