Improving PDF to ensure best possible "print on demand" results

My project is about improving a PDF file to ensure the best possible results in print on demand. I am not sure what the quality of the PDF is so I need assistance to ensure it is up to par. I am also unsure if I need any changes or modifications made to the PDF and I need expert advice to point me in the right direction. The intended size for printing is a 6" by 9" book. I need help making sure my PDF is optimized for this size and that I am getting the maximum quality possible in print on demand.