Graphics and text designs (basically, any non-photographic content) are best submitted as vector based PDFs. This will ensure your lines and text are nice and crisp. If you upload bitmap-based content (JPEGs or PNGs), there may be some fuzziness around your text.
You should also make sure that there are no cut or crop lines, all transparencies have been flattened, overprint is switched off and any fonts used are embedded, before saving your designs. For best results, we recommend using the following preset to preflight your designs: Adobe PDF/X-1a:2001.