I have a site A where is installed a web portal written in python. 
Then, I have a site X (that is not static but change dinamically), where are stored some file. The site A and site X communicate through ftp.
How can i allow a registered user of the portal  to download a file like the file was in the site A. Is there a standard way to do this? 
Since the files can be large I would avoid to pass for the server.
Thanks
The only way to grant access in the manner that you want to is to pass it through your server , write a frontend on the FTP server, or provide a limited download of the file on the FTP (Temporary account).  The latter option is not secure and wouldn't recommend it although it would be easy to do. So, that leaves either passing the file through your server and handing it off to the user that way or having some kind of web frontend on the FTP server to serve the file. 
The frontend on the FTP server would be the best option, although it requires more work, but the basic requirements are: 

Link generation
Database of some kind to hold the links/user's allowed to access it.
A method to pass the authentication to this frontend so the user doesn't have to relogin anywhere, simple cookie/session would be easiest but again is difficult.

It will require a lot of extra work but will be the most flexible, that is if it is possible to do this else I would stick with passing the data through or look into a third party CDN.
Make accessible a remote file to a registered user