I held off from replying straight away because I thought I'd fixed the problem by rebooting the PC. And search worked exactly as it should. (For me Nitro freezing when searching had been an ongoing issue with versions 12 and 11. I posted just after installing version 13.)
Then, just now, I loaded Nitro, opened a document, the same as last week, made a search, it displayed the results and and instantly froze. It doesn't matter what word or term I am searching for. You are welcome to test the document concerned but as the proof for a book it is quite large. Where should I send it?