$custom_field = get_post_meta($post->ID, "secim", $single = true);
   if(isset($custom_field)){
echo'
<div class="arsiv">
      <a href="'.the_field('pdf').'" target="_blank"><img src="'.the_field('resim').'"/></a>
         <a href="'.the_field('pdf').'" target="_blank"><h2>'.the_title().'</h2></a>
         <p>'.kisalt($post->post_content, 150).'...</p>
 
                <a href="'.the_field('pdf').'" target="_blank" class="pdf">PDF</a>
      <a href="'.the_field('solidworks').'" target="_blank" class="solidworks">Solid</a>
      </div>';
}else{
echo'
<div class="arsiv">
      <a href="'.the_field('pdf').'" target="_blank"><img src="'.the_field('resim').'"/></a>
         <a href="'.the_field('pdf').'" target="_blank"><h2>'.the_title().'</h2></a>
         <p>'.kisalt($post->post_content, 150).'</p>
      </div>';
}