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Your drawing is probably scanned. This process flattens it into one bitmap raster image which removes all the information about how the drawing was created or what it even is. As a result, it can not be redrawn or exploded into objects by pdf2cad. The most we can do with scanned files, is to create a high-quality copy to be used as a tracing layer in your CAD program.
If you look in the directory where the file was saved, you’ll notice that two or more files were created – a DXF and at least one TIFF or JPEG image. The DXF references the image(s) and assembles them on-screen when loaded. They must be in the same directory as the DXF file or the DXF file will look empty. This blank page effect will also occur if your CAD package does not support raster images.
If you want to try to create vector objects from a scanned drawing, you need a different class of software called “raster to vector”. This is an extremely difficult thing to do well. We recommend you start at trixsystems.com. They have a good product which performs specialized raster-to-vector conversion.