Yes I believe that sounds correct. Aspose.Words can open word documents and render them to .tiff files but it cannot load a PDF file in to convert to a .tiff. So in that case you would need to use Aspose.PDF as well to accomplish this.
The Aspose.Pdf kit is able to do this conversion from PDF to .tiff. If you need to ask any further questions regarding this conversion you can ask it on the Aspose.Pdf.Kit forum here.
To convert a document into an multipage .tiff file then you can use the following code below.
doc.SaveToImage(0, doc.PageCount, "Document Out.tiff", null);
Also please see the API documentation here for more information.
If there is anything else you would like to know regarding the conversion from a document to .tiff then please feel free to ask here.