Creative Upload Error: Not a valid PDFx1a file
This error indicates your creative files were not exported as Adobe PDF Preset: [PDF/X-1a:2001], which is our recommended format for best quality print. If PDF/X-1a is not used, you run the risk of inconsistent coloring and shadows or overlays printing as solid colors. Visit the link below for more details and export instructions:
Static Creative Guide
Update
PDF/X-1a is no longer the industry standard for print. Although it works well and is still widely supported, please export as PDF/X-4 wherever possible.