When certain programs (Indesign/Photoshop/other 3rd party editing tools) export to pdf, they sometimes have various layers which might show up fine in your pdf viewer, but might show up different on another pdf viewer, or more especially in the processing engine on the printer. This might lead to layers being moved to the wrong section or not showing up. So submitting as a single layer png or jpg will remove that potential issue. Note for some uploads (custom boxes, rules), we do allow for pdfs.