Search box does not work

I recently bought and downloaded a long, complex (and expensive!) text book which I really need to be able to search on. The search box in Adobe Digital Editions does not function for this book (rectangular search box upper right). Anything entered in the search box simple returns 'No match'.

It is a pdf, converted from acsm. I am using a Mac. There are one or two similar posts but I can't see any resolution for this anywhere.