That's a good point. Back when it was created, doing that with the toolchain wasn't so straightforward as I understand it is now; I'll raise it on the groff@gnu.org list.

You can generate PDF (without the hyperlinks) with standard psutils (e.g.: ps2pdf). A good PDF reader will offer text search, which is a bit of a help.

Otherwise, I'd have to investigate groff and text conversion utilities to see if the metadata are readily accessible. Interesting question, though I suspect it might take a slight rewrite.

PDF is provided on the book's page I gave above. You're correct, small mark-up changes are needed to make use of the PDF-linking macros now available that weren't then.