It actually depends on how you intend to use the resulting document. If you will be printing the resulting PDF on a professional press, then you will want to use a lossless format (most likely tiff), or a high quality jpeg.
If the PDF is to be put on your website, you will probably want to use a lossy format of some type. It will give a better quality to file size ratio, even though it will introduce some artifacts into the image. But unless your readers will be viewing the image at very high resolution (onscreen), they are not going to notice. Hope that's of some help. Cheers, Rob
