I am the author of the SolidEdge.Community.Reader GitHub repository and SolidEdge.Community.Reader NuGet package. The reader leverages embedded viewer data of the .DFT file. If you open a .DFT with the Solid Edge View & Markup application, the sheets you see there are what you will have access to via my reader. Solid Edge does not embed viewer information for background sheets so it's not possible to access them with my reader. Currently, the only way to access all of the sheets of a .DFT is to use the full Solid Edge API.