Hi
I just checked the issue which you said here. Infact, that's the way the extension work. The popup is used to show detail information of document in nice layout so that customers can see document's detail before deciding to purchase or download the document .
After viewing detail information of the document, customers can close the popup, and then purchase the document from document list page .
Hope my explanation is clear to you .
Thanks,
Tuan
PS : if you want to show the order button in the popup, you can get this file (I modified the original files a litle), unzip it and upload to components/com_dms/views/document/tmpl folder . After that, it will work as expected .