I have every interesting article or web reference I've ever read since 1997, sitting in a PDF file alongside tens of thousands of others.
It is so convenient to be able to "ls -l | grep subject" and find all the pages related to that subject, and then to mine the data out of the PDF's for further reference.
Or, I just open the PDF and gain the knowledge again.
This works very well and doesn't require an active Internet connection. Every year I spend a few weeks up in a mountain retreat, going through the collection and getting an idea for the variety of topics I've read about for the year.
Its pretty neat to see, also, the changes in my interests over the years - and as well, its pretty wild to go back to the sites after time and realise I've got the only copy of the site - because its gone now.
I'd prefer to see more tools for manipulating PDF's become mainstream in the future. I can't recommend highly enough the productivity I've gained by being able to organise things this way. Its like having my own private Internet of 80,000+ pages, tailored to all the things I'm interested in ..