3dm to pdf conversion represents export of 3D model / CAD file is to PDF format is possible and is usually done so the output is called 3D PDF file. This creates a rotatable 3D preview of the model on one of the pages of PDF document. A lot of 3D modeling tools nowadays have this function, but you may also use some dedicated 3D converter for it.
Open 3DM file Open PDF file3D CAD/CAM tool for product development
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A commercial online CAD converter