Unlike overgrown apple trees you have to be far more careful with pears.
Especially when they are getting on a bit and have reached great hights.
Usually in a garden it is best to leave them alone and just enjoy the blossom and harvest the few pears that you can reach.
However if you feel that something must be done then here goes.
How low you can safely bring the tree depends an awfull lot on how low fresh wood is being produced on the tree.
In general it is safe to cut down to good growing wood....but be carefull that that wood is still producing new timber and is not just sitting there with no new growth occuring.
If in doubt then resduce the tree slowly over a couple of years.
Very old pear trees can be very reluctant to re-structure if hit too hard.
An apple a day.....keeps me in work.