PDFs served from datocms-assets.com are currently showing up as image thumbnails rather than PDF files for Chromium-based browsers (Chrome and Edge). Firefox and Safari are not affected.
This is a bug on our end and we are trying to resolve it ASAP. Very sorry about this.
A current workaround is to append ?skip-default-optimizations to the end of the URL, like:
https://www.datocms-assets.com/123456/123456789-your-file.pdf?skip-default-optimizations
That should allow the PDF to load normally.
We are working on a permanent resolution and will update here as soon as we can.