Update your DOMPDF library, version 6 or higher is required!

Added a link for Drupal 7 to the dompdf 0.6-beta2 version at http://drupal.org/project/invoice.

Direct link: http://www.platinadesigns.nl/downloads/dompdf_0.6-beta2.tar.gz.
I added the Arial font to this one, just like the 0.5.1 version.

Pieter

This issue still exists. Due to the fact that you are checking for a string in the DOMPDF file and versions after 6.2(ish, some 6.2 betas seem to not work either) do not have that string, what you are actually checking is whether DOMPDF is after 6.0 but before 6.3, which makes your module hard to use, as 6.3 is readily available and is what other modules will expect.

To fix the problem, at least temporarily, I changed the file invoice_helpers.inc, function _invoice_dompdf_include_lib starting at line 727 (if ($file !== NULL) { ) :

original:

  if ($file !== NULL) {
    $content = file_get_contents($file);
    if (strpos($content, 'spl_autoload_register') !== false) {
     require_once $file;
    }
    else {
      drupal_set_message(t('Update your DOMPDF library, version 6 or higher is required!'), 'error');
      $error = TRUE;
    }
  }

Changed:

  if ($file !== NULL) {
      require_once $file;
  }

Here is the patch.