public function buildForm(array $form, FormStateInterface $form_state) {

  $form['#attributes'] = array('enctype' => "multipart/form-data");

  $form['pdf_file'] = array(
    '#type' => 'file',
    '#title' => $this->t('PDF File'),
    '#description' => $this->t('PDF files only'),
    //'#multiple' => TRUE,
    '#required' => TRUE,
  );

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Save'),
  );

  return $form;
}

public function buildForm(array $form, FormStateInterface $form_state) {

  $form['#attributes'] = array('enctype' => "multipart/form-data");

  $form['pdf_file'] = array(
    '#type' => 'file',
    '#title' => $this->t('PDF File'),
    '#description' => $this->t('PDF files only'),
    //'#multiple' => TRUE,
  );

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Save'),
  );

  return $form;
}

$( "#target" ).submit(function( event ) {
  if( $(".pdf-file-field").files.length == 0 ){
    alert("Please add a file");
    event.preventDefault();
  }
});

namespace DrupalmymoduleForm;

use DrupalCoreFormFormBase;
use DrupalCoreFormFormStateInterface;

/**
 * Class ExampleFileManagedForm.
 *
 * @package DrupalmymoduleForm
 */
class ExampleFileManagedForm extends FormBase {


  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'example_file_managed_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {


    $form['pdf'] = array(
      '#type' => 'managed_file',
      '#title' => $this->t('Image'),
      '#upload_location' => 'public://pdf/',
      '#upload_validators' => array(
        'file_validate_extensions' => array('pdf'),
      ),
      '#required' => TRUE,
    );

    $form['submit'] = [
        '#type' => 'submit',
        '#value' => t('Submit'),
    ];

    return $form;
  }

  /**
    * {@inheritdoc}
    */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    dump($form_state->getValue('pdf')); die();

  }

}

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  0 => "467"
]