I can add item number to the to do (there is a new version of the five filter pdf engine in the pipeline so going to wait until that is ready). In the meantime you can manually override by opening rss-make-pdf-newspaper.php in the plugin editor and replace
while( have_posts()) : the_post();
with (where posts_per_page is how many items you want to include)

$my_query = new WP_Query('posts_per_page=10');
while ($my_query->have_posts()) : $my_query->the_post();