The issue has been resolved.

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.

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.