When I was younger (!) and I had time, I used Calibre to unlock my e-books to be sure I will be able to read it (only for archiving purpose). Now I prefer sell

When I was younger (!) and I had time, I used Calibre to unlock my e-books to be sure I will be able to read it (only for archiving purpose). Now I prefer sellers who provide me pdf books. For this reason I prefer to buy technical books from O'Reilly or Packt: they cost more of the same book on Amazon, but I get multiple unlocked formats, and I can print some pages if I need to. It is okey for me to pay a bit more to get a future-proof format like a pdf one. PDF seems the only format which is a bit more durable nowadays. The 7-bit ASCII is probably the most "future proof" format we have: even a Commodore PET (1980) should be compatible with 7bit-ASCII as far as I know.