500 internal error while uploading a document

At first when I try to upload a document, right after I click on save…I get this

This page isn’t working
example.com is currently unable to handle this request.
HTTP ERROR 500

then I turned debug to true

Notice: Only variables should be passed by reference in /var/www/cf/wp-content/plugins/bsk-pdf-manager/inc/bsk-pdf-manager-pdf.php on line 336

Fatal error: Uncaught Error: Call to undefined function utf8_decode() in /var/www/cf/wp-content/plugins/bsk-pdf-manager/inc/bsk-pdf-manager-pdf.php:350 Stack trace: #0 /var/www/cf/wp-content/plugins/bsk-pdf-manager/inc/bsk-pdf-manager-pdf.php(310): BSKPDFManagerPDF->bsk_pdf_manager_pdf_upload_file(Array, 1, '') #1 /var/www/cf/wp-includes/class-wp-hook.php(286): BSKPDFManagerPDF->bsk_pdf_manager_pdf_save_fun(Array) #2 /var/www/cf/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array) #3 /var/www/cf/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /var/www/cf/wp-content/plugins/bsk-pdf-manager/bsk-pdf-manager.php(205): do_action('bsk_pdf_manager...', Array) #5 /var/www/cf/wp-includes/class-wp-hook.php(286): BSKPDFManager->bsk_pdf_manager_post_action('') #6 /var/www/cf/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #7 /var/www/cf/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #8 /var/www/cf/wp-settings.php(450): do_action('init') #9 /var/www/cf/wp-config.php(89): requi in /var/www/cf/wp-content/plugins/bsk-pdf-manager/inc/bsk-pdf-manager-pdf.php on line 350

I am not using the pro version at the moment, but using the basic version already giving me this error. Is there some settings that I need to do?