I have found a view apps, which support sending messages by fax. Is there a special framework available for that? How do I implement fax support in my app at all? I am trying to get some info about that, but the only thing I find are finished apps but no clue how to implement such a feature correctly.
If it is not possible directly, what would be a way to provide fax support? Is it possible to connect a fax with bluetooth to an iPad and send a PDF to the fax?
This is the sort of thing that is best handled by a third party service. Try searching on "fax api" and you'll find a number of companies that provide APIs that allow you to send and receive faxes via a web service.