For more information see the PDF called "Understanding PDFs when working with CtrlPrint" here.
The quality of images will vary depending on who last uploaded a chapter to the server. InDesign users who have access to the high quality images will produce PDFs that reflect this, while InCopy users will produce PDFs with low quality images.
If you need a PDF with high quality images contact your agency who can create one for you.
In addition to image quality, the fonts used in the PDFs will vary if users do not have all of the included fonts installed.