WordPress meta query not working

When using LIKE clause, you should be using % to the start and end of the value to make it match with the existing values. So your query should look like this.

add_action( 'pre_get_posts' , 'my_pre_get_posts' );
function my_pre_get_posts( $query ) {
    $value="%".$_GET['s'].'%';
    if( $query->is_main_query() && $query->is_search() ) {
        $query->set( 
          'meta_query',
          array( 
            array(
              'key' => 'pdf_content',
              'value' => $value,
              'compare' => 'LIKE'
            )  
          ) 
        ); 
     }
}