The preview stream can be generated using PDFNet SDK (http://www.pdftron.com/pdfnet/downloads.html) as shown in SilverDoxStreaming and SilverDoxConvert samples. The stream does not need to be written on disk although this could in many cases improve the performance.
PDF format is only one of formats supported by SilverDox Publisher (i.e. PDFNet SDK or DocPub CLI). For historical reasons, the use of PDF on the web was really an afterthought and there are many reasons why it is not an ideal Web format. SilverDox was designed from ground-up for web use and to eliminate many PDF weaknesses. For example, most PDFs on the web are not properly linearized and this feature never really delivered on its promise. In addition the whole scheme is still clouded with patent issues - with Adobe still specifically excluding patent right for 5,860,074 on their website: http://partners.adobe.com/public/developer/support/topic_legal_notices.html. In addition reliable/accurate/efficient implementation for a fully compliant PDF support in browser is almost mutually exclusive and would not work in managed environments required for the next generation of web apps (Silverlight, Flash, or HTML5). There are also many other issues (such as corrupt or unoptimized files, press files with large images etc). For all of these reasons there are currently no non-native (i.e. plug-in) solutions that are fully compatible with ISO PDF, and this will not change anytime soon. So solutions claiming full PDF support (Silverlight, Flash, or HTML5) are simply misleading users.