We do not recommend adding direct attachments to e-mails, as otherwise the e-mail will become very large and won't be accepted by some recipients or will maybe be blocked by spam filters.
Our suggestion is therefore to send only links via the thank you e-mail (e.g. to your trade fair brochure). Links can be maintained via the questionnaire and are only integrated into the thank you e-mail if a specific answer is chosen.
If you still want to send a classic attachment with the thank you e-mail, you must first import it via Settings → Email → Email standard text (e.g. via iCloud, Dropbox). You must then refer to this file name in the questionnaire (see screenshot). If you want the same attachment for all your devices, this can only be done via link (see above) and with a configuration profile.