A print-ready PDF file needs to have the following properties.
CMYK: All text, images and other items need to be in CMYK (not RGB). The resolution of the images needs to be 300dpi or similar.
Fonts: All fonts must be embedded or outlined.
Bleed: If any items go up to the edge of any of the pages there needs to be a 3mm bleed on the document. The items going up to the edges need to fill the bleed area.
Safe area: This applies to text or any other important item such as a logo. These should not be positioned closer than 3mm from the edge of the page. This is again to allow for variation in printing and guillotining.