In Teigha you can work with solids and shells using boundary representations, or B-Reps. In different Teigha products, Teigha Drawings (for working with both .dwg and .dgn files), Teigha PRC, and Teigha BIM, there are separate implementations of B-Rep interfaces, but you can work with them using the common class OdBrBrep.
With released Teigha 2019, ODA members have a new way of importing .pdf files as underlays using Teigha’s new PdfiumModule module. The module is based on the Pdfium library developed by Google. Pdfium is available as open source and free of charge. Previously ODA members could import .pdf files as underlays using the PdfVIModule module but at an additional cost.