Thanks for using GroupDocs.Assembly for .NET. Currently, PDF file format is not supported by the API, please have a look at the Supported File Formats .
Currently, using GroupDocs.Assembly API you can use word document as a template and save it as presentation and other supported file formats. You can see an example here.
Furthermore, the blog post you have shared above is related to an obsolete version of GroupDocs.Assembly. We have now released the next generation GroupDocs.Assembly API which is a UI less, backend API for document automation and report generation. For more details about the next generation API, please have a look at the documentation. You can also evaluate all the features of the API using the examples project hosted here. Hope it helps.