Go to the 'Project PDF' section. There should be a link (in blue) something like Book_Name.pdf.
You can just click on it, or you can "download" it. There is a big difference.
If you click, the PDF will probably open inside your browser window
To download, right-click (Ctrl click on a mac) on the PDF link and choose "Save as . . ." from the list of options. Navigate to the location where you want to store the file and save it there. Open the PDF from your storage location.