Issues with PDF Previews on File Server After Update

This issue seems linked to the security changes Microsoft made regarding preview handlers for files from network locations. Windows has started classifying files opened via UNC paths as less trustworthy, which may block the PDF preview. Check if the PDFs carry a Mark-of-the-Web or are treated as from the Internet zone. Adjusting Group Policy for preview handlers in protected view might help, or try adding the file server to the Local Intranet zone through GPO.

You might want to try accessing the file server using its Fully Qualified Domain Name (FQDN) or a different format in the UNC path. For example, you could experiment with 'file://fileserver', 'fileserver', or 'filserver.domain.local'. There’s also a helpful Microsoft article about how the preview feature can become disabled for files downloaded from the internet, which might be relevant to your issue.

I’ve been grappling with this problem for a while. The best solution I found was to implement a Group Policy that prevents blocking PDFs when accessing them via UNC paths. Additionally, running a script to unblock files several times a day has been a temporary fix for issues with attachments dragged from external sources.

If local access works but UNC access doesn’t, it’s likely that the preview handler is getting blocked due to network security settings. Check your Internet Options under Security to ensure that the server is listed in the Local Intranet sites. Also, look into Group Policy settings that might affect how file attachments are treated when they come from network locations.