If the attachment in the db is a tiff, I believe the API will return the tiff. The fax manager app is accessing the attachment directly in the DB, when a fax is received it is saved to the db in tiff format. Teletype does convert to PDF which should work if libtiff-tools is installed, but only in email notifications. I don't believe it converts it in the API returning it though, this wouldn't be very difficult to implement, it would require a UI update to modify the request to ask for PDF and the api would need to convert to pdf before returning the file.