IMO, that's foolish of the printer. I can't think of any reason to prefer a PDF.
PDF is just a container for the image. At best it would maintain the image exactly as-is, at worst - it could optionally re-encode or re-compress your image based on the PDF encoder's settings.
Like Save for Web there are compression settings. Make sure you set those appropriately. If you don't want any loss of data than choose "None" or "ZIP" under the compression settings.
