Airtable Support says:
Hi Matthias,
Thank you for reaching out about the access issues with your public attachment links.
The links generated via v5.airtableusercontent.com are expiring download URLs, which means they will expire after a short period (at least 2 hours) for security reasons. This could be why users are having trouble opening or downloading the PDF files on your Softr site.
To ensure persistent public access to your files, I recommend hosting them externally. Unfortunately, Airtable is not designed for permanent public file hosting.
If you have any further questions or need assistance with setting up an external host, feel free to ask!