Find if a library exists in a Drupal path recognisable by Libraries module without needing libraries_load() or an implementation of hook_libraries_info(). This code is looking for the MPDF library.

drupalModuleLibraryExists.php

<?php
/**
 * Function to check existence of mPDF library.
 *
 * @return bool
 *   TRUE if mPDF library path exists and FALSE if it isn't.
 */
function module_mpdf_exists() {
  // Search for mpdf tool first.
  $pattern = '/^mpdf.php$/';

  // Use the libraries module to detect mPDF library in case of multisite
  // installation.
  $tools = array_keys(file_scan_directory(libraries_get_path('mpdf'), $pattern));

  if (isset($tools[0])) {
    require_once $tools[0];
    return TRUE;
  }
  else {
    return FALSE;
  }
?>

drupalModuleLibraryUse.php

<?php
require_once 'drupalModuleLibraryExists.php';

function module_mpdf_save_to_file($html) {
  $uri = FALSE;
  if (!module_mpdf_exists()) {
    return $uri;
  }
  // The default format is A4, A4-P for portrait.
  $format = 'A4-P';
  // Make a new object.
  $mpdf = new mPDF('UTF-8', $format);
  $mpdf->SetCreator(variable_get('site_name', 'Drupal'));
  $mpdf->WriteHTML($html);
  // Try to recover from any warnings/errors.
  ob_clean();
  // Prepare the file to be saved.
  $filename = "myCustomPDFFile.pdf";
  $uri = file_stream_wrapper_uri_normalize('public://pdfs');
  file_prepare_directory($uri, FILE_CREATE_DIRECTORY | FILE_MODIFY_PERMISSIONS);
  $uri = "{$uri}/{$filename}";
  $pdfoutput = $mpdf->Output('', 'S');
  file_unmanaged_save_data($pdfoutput, $uri, FILE_EXISTS_REPLACE);
  return $uri;
}