Alternatively you can:
- upload your pdf file in the orphaned activities as "file" (there are a number of tutorials on Youtube on orphaning activities in moodle).
- Choose "show" (but it will remain invisible to students, as it will be in the orphaned activities).
- Copy its link and paste it into this HTML code, substituting the red string <center><embed src="yourdomain.com/mod/resource/view.php?id=443#toolbar=1&navpanes=1&scrollbar=1" quality="low"height="540" width="720" /></center>
- Copy the resulting new HTML code and paste it into your Book page (using the HTML tab).
If your pdf file is already on the web, just copy its link and paste it as above - no need to place it in the orphaned activities.
Hope it helps!