Resize PDF with WP_Image_Editor to square JPEG thumbnail.

gistfile1.php

<?php

// Load PDF
$img = wp_get_image_editor( 'my/temp/uploads/path/mypdf.pdf' );

// Resize PDF
$img->resize( 50, 50, TRUE );

// Generate filename
$filename = $img->generate_filename( 'thumb', 'wp/wp-content/uploads/thumbs/', 'jpg' );

// Save
$img->save( $filename, 'image/jpeg' );

?>