Bring boiling-water canner, half full with water, to simmer. Wash jars and screw bands in hot soapy water; rinse with warm water. Pour boiling water over flat lids in saucepan off the heat. Let stand in hot water until ready to use. Drain well before filling.
Peel and core pears. Finely chop or grind fruit. Measure exactly 5 cups prepared fruit into 6- or 8-qt. stockpot. Stir in lemon juice.
Mix 1/4 cup of the sugar (from the measured amount in the bowl) and pectin in small bowl. Add to fruit in stockpot; mix well. Bring to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly. Stir in remaining 2-3/4 cups sugar. Return to full rolling boil and boil exactly 1 min., stirring constantly. Remove from heat. Skim off any foam with metal spoon.
Ladle immediately into prepared jars, filling to within 1/4 inch of tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly. Place jars on elevated rack in canner. Lower rack into canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.) Cover; bring water to gentle boil. Process 10 min. Remove jars and place upright on towel to cool completely. After jars cool, check seals by pressing middles of lids with finger. (If lid springs back, lid is not sealed and refrigeration is necessary.) Jam sets slowly and may take up to 1 week to set completely.