To perform a search in any of the 3 PDF family screens in this section, you will need to use the "popout" button at the top right of the PDF screen.
See image below. To see the popout you may need to put your cursor inside the document area and if necessary click inside the document. You then will see the popout. Click on it and it will take you to another screen where you can use a Ctrl F on the keyboard to display a search box.
The search screen and search box will look like the image below.