Editing the pre_get_posts will replace the original query and you will not have the content for your page at all. I would only recommend going this approach if you only wanted to display the content of your portfolio post type and not the content of your portfolio page.
For general post queries it is recommended to use WP_Query or get_posts.
http://codex.wordpress.org/Class_Reference/WP_Query
http://codex.wordpress.org/Template_Tags/get_posts
If you use the WP_Query function the wp_reset_postdata() will restore the post data back to the original so you can get the content of your original page.
$args = array(
'posts_per_page' => 10,
'post_type' => 'portfolio',
);
// The Query
$the_query = new WP_Query( $args );
// The Loop
if ( $the_query->have_posts() ) {
while ( $the_query->have_posts() ) {
$the_query->the_post();
echo '<li>' . get_the_title() . '</li>';
}
} else {
// no posts found
}
/* Restore original Post Data */
wp_reset_postdata();
Now you will be able to use the original loop to show the content of your page
<?php while ( have_posts() ) : the_post(); ?>
<?php the_content(); ?>
<?php endwhile; // end of the loop. ?>