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using AcadApp = Autodesk.AutoCAD.ApplicationServices.Application;

//open .dxf file
AcadApp.DocumentManager.Open("somefile.dxf", false);

//do some actions

//need to save as .dwg 2004
var doc = AcadApp.DocumentManager.MdiActiveDocument;
doc.Database.SaveAs("file.dwg", DwgVersion.Ac1800);


/*
The problem is that after opening saved file Autocad 2012 (version 18.2) says that some errors've
been found and offer to recover file. After that everything is OK. If I save as DwgVersion.Current 
or DwgVersion.Newest everything is Ok too.
*/