How can I migrate to latest vtiger CRM version with installed PDF Maker extension ?

1. Please Deactivate your License key before mirgation
2. After successful migration to latest vtiger CRM version, our extension is disabled
3. Go to Setting->Module manager and choose Custom Modules tab (for vtiger CRM 5.x)
4. Find our PDF Maker extension and enable it
5. Download our compatible release version with your vtigerCRM version from our download center
6. Upgrade our extension using Modul Manager and follow installation steps
7. In case of some problem with license key, click on “Reactivate license“ button. You will be requested to insert your license key and your license should be reactivated

For more informations, please visit https://www.its4you.sk/en/pdf-maker/manual/21-pdf-maker/manual/88-license-settings

After installation I got folowing error:
Error - mPDF requires mb_string functions. Ensure that PHP is compiled with php_mbstring.dll enabled.

PDF maker uses MPDF as PHP class to generate PDF files from HTML. MPDF requires the mb_string module to be enabled (see http://uk.php.net/manual/en/ref.mbstring.php) which means that PHP>=4.3.0 will be required as a minimum, and PHP 4.3.0 to 4.3.3 need --enable-mbstring=all. For Windows users: Enable the extension(s) in php.ini you want to use by uncommenting the extension=php_*.dll lines in php.ini. This is done by deleting the leading ; from the extension you want to load and restart Apache. http://php.net/manual/en/install.windows.extensions.php
// change the following line from ...
;extension=php_mbstring.dll
// ... to
extension=php_mbstring.dll

System requirement

• PHP up to 5.6 (we do not recommend PHP 7 because vtiger is not fully compatible with PHP 7 and you can have more issue with using)
• max_input_vars 6000
• max_execution_time 30000
• memory_limit = 512M
• post_max_size = 128M
• max_input_time = 120