If you book a day tour ticket using a cell or ipad while there,, no printer. How is the booking ticket handled by the various tour companies? I suppose they send you back a confirmation. Can you just show that to the driver? No printout. And rather than lug an ipad along... maybe there is a book number such as LZ1234. Just verbally tell the driver that number and all set to go? Any first hand insight is most welcome!
All in business spell out the rules on the website ( or in the confirmation email) where you order what you need to present (in print or electronically) when show up. That is my experience.
At various attractions we went to in Scotland, we saw scanning of iphones to be very common.
It depends on what the company requires. If they instruct you to bring a printed copy, you should bring a printed copy. Usually your the front desk at your hotel (or the owner at your B&B) will be willing to print for you. Just email them the email or PDF.