Hi there, Lars! This is Marc. Thanks for providing those details. Based on what you've described, here are a few approaches we can try.
Check if you have a PDF reader installed and set as your default application for PDF files. If you don't have Adobe Reader, PDF-XChange, or another PDF viewer installed, Windows might not know how to handle the files. You can download Adobe Reader for free or set Microsoft Edge as your default PDF viewer through Windows Settings > Apps > Default apps. Also, make sure your PDF reader is up to date, as older versions sometimes have compatibility issues with attachments.
If the PDF reader isn't the problem, the issue might be Outlook's attachment security settings. Go to File > Options > Trust Center > Trust Center Settings > Attachment Handling, and make sure PDF files aren't listed in the blocked attachments.
Let me know how it goes.
Best regards,