How do I convert a Keynote file to a high resolution PSD?

I mocked up several marketing materials for my team in Keynote, and I'd like to convert them into high res files (preferably PSDs or JPGs) to upload to a printing site (like Vistaprint). I've tried to convert to PDF and JPG directly from Keynote, but the files come out pixelated and at a much lower resolution when uploaded to the printing site.

I'm hoping I can solve the pixelation upon upload by converting my Keynote files to PSDs (if at all possible).