Why DXF files differ so much
DXF is a documented format with a long history, and every CAD package writes a slightly different subset of it. A drawing from a sheet-metal nesting tool and a drawing from a survey package can share an extension and almost nothing else.
Binary DXF exists too, and is common in machine output because it is smaller and faster to parse. Plenty of viewers only implement the ASCII form and simply reject the rest.
Converter sites versus local parsing
Uploading a DXF to a free web converter usually returns a flat raster or a PDF. You lose layers and blocks, which is the structure you actually wanted, and the drawing has been handed to a third party.
Local parsing keeps the structure and keeps the file. For a fabrication drawing under an NDA, or a client's site data, that distinction is the whole decision.
What CadRoam reads and what it does not
Supported: model-space linework, curves, blocks, layers and mesh faces, from ASCII and binary DXF, parsed on device.
Not supported: editing, dimension authoring, plotting to a titleblock, and certified measurement. Paper-space layouts and exporter-specific records outside the supported scope may render partially, and the app names that rather than hiding it.