Exporting the Print Ready PDF
If you have already upgraded to macOS Mojave 10.14
In the top right hand corner of the Apple Photos window, click Export PDF to Export to PDF in Mojave
The process is slightly different with each prior MacOS Photos or iPhotos version:
Photos in MacOS 10.13 - Click File > Export > Export book as PDF and click Export Production PDF
Photos in MacOS 10.10; MacOS 10.11; MacOS 10.12 - Option+Click on Buy Book
Photos in MacOS 9.6.1 - Option+Click on Buy Book or Right-click on the pages and choose Save book as PDF
Note: If you have trouble exporting a PDFs out of Apple Photos or iPhoto, and are not using the Mimeo Photos service to build your project, please contact Apple Support to get help for Apple Photos.