Need photos. But possibly an air-borne fungus called Quince Blight (pears and quince are related).

The only treatment is to collect and burn the affected foliage in the autumn to reduce the pathogen in the vicinity. You can then use Bordeaux Mixture sprayed on the leaves during the growing season to prevent further infection.

Might be Pear Rust, but without photos, hard to say. As far as I know, Bordeaux mixture has been withdrawn from sale.

there is also a pear leaf blister mite that produces brown discolouration. pick up and don't compost any fallen leaves. Pick off the worst affected leaves. A friend has this problem and over the last 3 years of picking affected leaves off she has finally got it under control. According to the RHS its unsightly but doesn't affect the fruiting. and there is no effective chemical control.

but as already said a photo is needed really.