Previewing is available only for Premium users. Free users can download but not preview documents.
Here is how Previewing looks for premium users:
It should also be noted that clicking Preview on a document will only work for PDF documents. If your documents in not a PDF, you will be prompted to convert it to PDF before viewing it.
The speed at which the PDF viewer opens will depend on the size and document quality. The larger the PDF, the longer it will take for the PDF viewer to open.
Documents currently cannot be embedded on a third-party website and can only be viewed through the internal interface.