The document preview tool is powerful and can interpret a wide variety of file types, however, it is not all-powerful. There are some things you can do to your files that will foil the preview generation and leave it hanging. The most common one we see, is adding security features to a pdf.

 

Just about any level of security, from a password lockout, to restricted copy and paste functionality on a pdf will prevent the preview generator from being able to process the file. This isn't a bug, as it is exactly what the document security is intended to do; it would be a little suspicious if our software could bypass the security. 

 

If you would like to check if your pdf has security enabled, the easiest way to do it, is to open it in Acrobat Reader. From there, click on the "Edit" and then on "Protection" in the drop-down menu, and finally on "Security Properties." 

 

If the document has anything but "No Security" in the security properties, a preview cannot be generated.