GSS converted to an all online product early this millenium.
If you need hard copies of the books, each chapter of the book is a separate web page and at the top right corner of each chapter there is a "Print Friendly" button. In order to download the entire book as a PDF, you may print to PDF, chapter by chapter. We find that most teachers like to create a customized course, not using
all the chapters, so this method works well for the most part.
If you really need a single PDF of the entire book, you can use an application such as Adobe Acrobat to construct the whole book as PDF.
Not relying on hard copy publication also allows us to update chapters on occasion and to have updates immediately available, not having to wait to produce a "new edition whole book" every 3 years.