Dimitrios, if you password protect the download page that could be used to provide the password when they register?

You could also make a contact form. In the automatic answer to the client, you could specify the link to the file.

Then the link will only be given to a "valid" email-address. In the form you can have name mandatory.

Of course the user can specify his name as Donald Duck, but he will only receive the link if the mailaddress is existing.