If you look on the product pages (the web page where you order), you will usually find the number of pages in a PDF product. However, we don't have all the web pages updated yet, so please email us at support@welltrainedmind.com and we will get you the info you need.
In some cases, you will to print all (or most) of the pages in the PDF. In other cases, you pick and choose--this is particularly true with the Student Pages from The Story of the World Activity Books.