1. I want to add more fields than the existing ones (about 20) to characterizer each document record. I can create these columns in Mysql table but then is it possible to connect them to the interface (e.g. document editing)
=> Yes, it is possible if you know Joomla programming / coding. Basically, you will need to add fields to #__edocman documents table to store data, then edit code in the add/edit document form to add the fields you want to the form.
2. Can I include these fields in search plugins so I can search for keywords at selected field only?
=> Yes, but again, you will have to edit code in search plugin.
3. Is Edocman multilingual ? I want to build the system in Greek Language.
=> Yes.
4. Is the license connected to a specific website or I can use it to more than one website?
You can use it on un-limited of your websites.
Regards,
Tuan