You need the K2 article-id delivered via GET to your script. You can temporary write the data ($this->items->extra_fields) in a database table and read it in. Or you build a query and more to take it out of the K2 tables. This is not so easy. Or your send the fields data via GET, but then think about security and filter the data.

Temporary write the data ($this->items->extra_fields) in a database table and read it in. How so?

Can you use Yiota's example and try something like this
$yourvar = JRequest::getInt('id');
right where you are in the code and use it on the fly?

I changed the print button code on item.php on the directory \joomla\components\com_k2\templates\default.
Does this answer your question?

How can i be able to load and use the k2 classes inside my pdf file (external file)? If i could do this i woudn´t need to use GET or POST and make my website vulnerable