This is especially useful when I am doing an End of Module assignment on my Science course where I have to use multiple books to answer questions.
Start by making sure that all of the PDF files that you want to search are in a separate folder/directory.
Now open up one of them.
Go to Edit -> Advanced Search
Now select "All PDF Documents in" then click on "Browse for Location..." and search for the folder where the files are.
Now type the word or phrase that you are looking for and then click "Search".
You will then see a list of results. Click on the different files (books) and then it wil show you a list of all the instances in that book. If you click on any of the results, the book is then opened up in that location.
