The actual electronic version is a web based tool with a bunch more features
like location search, route mapping and joined up diagrams.
This requires a specific user account.
The downloadable PDFs are just like a paper document, splitting by region and page by page.
The pdf versions are currently freely downloadable.
At least I think that's how it works.
I haven't got access to the electronic version.
You need a Network Rail Portal account and I haven't got one.
Though I think certain outside organisations can get one if they apply,
because there are some third party tools using the data.
Edit: The setup makes sense, a few PDF downloads won't break the bank, but general access to a complicated GIS system is going to require more resources.