Prepare a boiling water bath canner and enough jars to hold approximately 3 pints of product. A combination of pints, half pints, and quarter pints is fine. Wash new lids and rings in good condition in hot, soapy water. Set aside to dry.
In a large, heavy-bottomed pot, combine chopped pears and sugar. Cook over high heat until the fruit can easily be smashed with the back of a wooden spoon. Use a potato masher or immersion blender to break the fruit down into a mostly-smooth sauce.
Add cinnamon and lemon juice and stir to combine. Reduce heat to medium and cook until the jam looks thick and sticky. Depending on the width and depth of your pot, this will take between 25 and 45 minutes. It will cook more quickly in a wider pan. This jam has a tendency to splatter a little while cooking.
When you have determined that your jam is finished cooking, remove the pot from the heat. Funnel the finished jam into the prepared jars, leaving approximately 1/4 inch headspace. Wipe the rims clean of drips, and apply the lids and rings.
Process the jars in a boiling water bath canner for 10 minutes (starting timer once the water has returned to a boil and not before). When processing time is up, turn off the heat, remove the lid and let the jars cool in the water for five minutes.
When that time is up, remove jars and set them on a folded kitchen towel to cool.When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.