by Michal Novák.
I am using mPDF and somethink like this $html = file_rewrite_pluginfile_urls($html, 'pluginfile.php', $file->contextid, 'mod_lesson', 'page_contents', $file->itemid);
Html is generated ok with images but when mPDF generates PDF it shows that error and don´t create PDF because i think that has not access to pluginfile.php and on that it fails