Hi Jeff,
Thank you for using Syncfusion Products.
Currently it is not possible to place the document pane window at bottom side like tabbed windows in Docking Manager by directly, but it is possible by some workaround.
We have prepared a work around sample for your requirement and you can download the sample from the attachment.
In this sample, we have placed one dock window at top (By SideInDockMode="Top") with NoHeader value is true. If NoHeader values is true for particular element means, then nothing is displayed in the header position and it is looking like an empty window. By this, the document pane automatically moved to bottom position and it looks as tabbed window.
Please let us know in case of any queries.
Regards,
Muthalagu.
BottomDocumentPaneInDockingManager_7882dbdf.zip