You can embed a PDF inside your book. When you click on the PDF, it will open inside Book Creator so you can read it whilst staying in the book.
There are numerous ways to do this, depending on which device you're using. The simplest way is to import a file into Book Creator.
From the + button, go to Media > Files and choose, for example, a PDF. You'll choose whether you want to add the PDF pages directly to the pages of your book, or embed it as a popup.
Choose Embed PDF as a link and this will embed the document onto your page like below. In Read Mode, you can click on this document and it will open up the PDF.
To learn how to import individual pages from a pdf please read 'Import your PDF into Book Creator'.
In Book Creator for iPad, the process for embedding documents is slightly different. Please read our article 'Share content to Book Creator for iPad from your cloud drives'.