Extra text field for Status order in Commerce I do have extra order status (Available for download) in Commerce. And when I choose this status I want to add a link to download a file. How can I add extra field for the order?

Probably, I even need two fields: "link" for the link and "text" for the description of link.
Consider using the Conditional Fields module. Excerpt from its project page:

Conditional Fields allows you to manage sets of dependencies between fields. When a field is “dependent”, it will only be available for editing and displayed if the state of the “dependee” field matches the right condition.
When editing a node (or any other entity type that supports fields, like users and categories), the dependent fields are dynamically modified with the States API.
You can, for example, define a custom “Article teaser&quot; field that is shown only if a &quot;Has teaser&quot; checkbox is checked.