Thanks for using GroupDocs.Viewer.
In case you can get the combined Html representation of all the documents into a single Html file then you can also get the PDF representation of that combined Html file using GetPdfFile method. You will use the Html file as the source file in GetPdfFile method.
You can also use Aspose.Pdf that is a product offered by our sister company. It allows you to concatenate the multiple PDF documents into a single document. For details, please visit this article. Hope it helps.