PDF files can contain raster images in a wide variety of formats. Each application which creates a PDF file may save the image data in their own way.
Scan2CAD supports the most common image encoding methods but you may find some PDFs which do not use a common method.
If you have a warning message stating that a PDF file contains an image, which is not supported, it will be because the image is encoded in an unsupported method.
Please feel free to contact us if you see this message providing the PDF. We will be happy to investigate the PDF to see if a solution is possible.