Several factors drive the persistent search for these books in PDF format:

In most of the world, copyright lasts for the life of the author plus 70 years. Catherine Aikman's exact dates of birth and death are not widely documented, which is part of the problem. If she died after 1954 (which is likely, given her active writing period), then her works are almost certainly still under copyright in the UK, the EU, the US, and elsewhere.

The series gained traction precisely because of its PDF availability:

Wait, Troy Aikman wrote a book called "Just Mercy" with David Garrow? No, that's Bryan Stevenson. Wait, Troy Aikman has a book called "Just Mercy: A Lawyer's Journey to Justice" but I'm not sure. Maybe it's a different title. Alternatively, maybe the user is referring to a different Aikman. Let me verify.