Program to create thumbnail

Hello. Prompt how to create a preview of the PDF file is downloaded to their handler.
sample code

<form method="post" action="<?php bloginfo('template_url'); ?>/handler.php" enctype="multipart/form-data" class="form-horizontal">
 	<?php wp_nonce_field( 'pdf_files', 'fileup_nonce' ); ?>

	
	<input name="pdf_files" id="uploades" required type="file" class="custom_media_url"/>
	<input name="xsq" class="get-val" value=""/>
 <button id="send" name="send" class="btn btn-primary">Отправить</button>
</form>

and handler.php

if(isset($_POST['send']) == '1') {
 $new_post = array(
 'ID' => '',
 'post_author' => $user->ID,
 'post_category' => 'portfolio-party',
 'post_title' => $post_title,
 'post_status' => 'publish'
 );
$post_id = wp_insert_post($new_post);
if ( ! function_exists( 'wp_handle_upload' ) ) {
	require_once( ABSPATH . 'wp-admin/includes/file.php' );
}

$uploadedfile = $_FILES['pdf_files'];
$upload_overrides = array( 'test_form' => false );
$movefile = wp_handle_upload( $uploadedfile, $upload_overrides );
if ( $movefile && !isset( $movefile['error'] ) ) {
	$filename = $movefile['file'];
	$parent_post_id = $applicant_id;
	$wp_upload_dir = wp_upload_dir();
	$attachment = array(
		'guid'           => $wp_upload_dir['url'] . '/' . basename( $filename ), 
		'post_mime_type' => $filetype['type'],
		'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $filename ) ),
		'post_status'    => 'inherit'
	);

	// Insert the attachment.
	$attach_id = wp_insert_attachment( $attachment, $filename, $parent_post_id );
	//$attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
	wp_update_attachment_metadata( $attach_id, $attach_data );

	// Update File Field
	update_field('pdf_files', $attach_id, $post_id);
} else {

}

I tried to add the code in this topic, but not work.
https://www.ads-software.com/support/topic/acf-front-end-editor/

I suspect that your plug-in only if the file is saved in admin’s part.

If anything, I’m sorry for my english.